Introduce ConnectionPool with master discovery

[DifferentialOrange]

python: drop Python 2 support

Python 2.7 reached the end of its life on January 1st, 2020 [1]. Since
it would be a waste to ignore several Python 3.x features in master
discovery implementation, we decided to drop Python 2 support here.

Python 2 workaround cleanup activities are expected to be solved as
part of #212 solution.

1. https://www.python.org/doc/sunset-python-2/

connection: introduce common interface

Introduce connection interface to be used in connection pool
implementation. Only CRUD and base connect/close API is required
by the interface.

Part of #196

connection_pool: introduce connection pool

Introduce ConnectionPool class to work with cluster of Tarantool
instances. ConnectionPool support master discovery and ro/rw-based
requests, so it is most useful while working with a single replicaset of
instances. ConnectionPool is supported only for Python 3.7 or newer.
Authenticated user must be able to call box.info on instances.

ConnectionPool updates information about each server state (RO/RW)
on initial connect and then asynchronously in separate threads.
Application retries must be written considering the asynchronous nature
of cluster state refresh. User does not need to use any synchronization
mechanisms in requests, it's all handled with ConnectionPool methods.

ConnectionPool API is the same as a plain Connection API.
On each request, a connection is chosen to execute this request.
Connection is selected based on request mode:

• Mode.ANY chooses any instance.
• Mode.RW chooses an RW instance.
• Mode.RO chooses an RO instance.
• Mode.PREFER_RW chooses an RW instance, if possible, RO instance
  otherwise.
• Mode.PREFER_RO chooses an RO instance, if possible, RW instance
  otherwise.
  All requests that are guaranteed to write (insert, replace, delete,
  upsert, update) use RW mode by default. select uses ANY by default. You
  can set the mode explicitly. call, eval, execute and ping requests
  require to set the mode explicitly.

Example:

pool.call('some_write_procedure', arg, mode=tarantool.Mode.RW)

Closes#196

[Mons]

Never look at box.cfg.read_only. Look only at box.info.ro

[DifferentialOrange]

Never look at box.cfg.read_only. Look only at box.info.ro

It was used only in tests, reworked

[DifferentialOrange]

Alternative approaches

Synchronous on request (on errors)

Solution idea: refresh schema (rw/ro info, replication state) on connect, RO error or network error.

Pros

• Since connector is synchronous itself, no new complicated mechanisms will be introduced.
• Retries due to schema change are easy to implement and effective.

Cons

• If no errors (for example, if we use only selects), schema will never be refreshed.
• Reloads block requests, thus latency for some of them will be long.

Synchronous on request (with timeout)

Solution idea: refresh schema (rw/ro info, replication state) before request if X milliseconds have passed since last refresh.

Pros

• Since connector is synchronous itself, no new complicated mechanisms will be introduced.
• Schema will be refreshed even if no errors on requests (for example, if we use only selects).

Cons

• No native retries on ro/rw or network errors. If we send RW request to RO instance, request must be retried for a full timeout before it succeeds.
• Reloads block requests, thus latency for some of them (one request each X milliseconds) will be long.

Asynchronous

Solution idea: refresh schema (rw/ro info, replication state) in separate thread each X milliseconds.

Pros

• Reloads are non-blocking for requests.
• Schema will be refreshed even if no errors on requests (for example, if we use only selects).

Cons

• Since connector is synchronous itself, we will need to introduce async mechanisms. Schema refresh will require x2 connections (otherwise synchronization primitives for x1 connections, which leads to requests blocking).
• No native retries on ro/rw or network errors. If we send RW request to RO instance, request must be retried for a full timeout before it succeeds.

What solution should we choose?

Solutions may be hybrid (1+2 or 1+3), and I think it would be the best approach to cover more cases. Personally I prefer synchronous on request on error + timeout: the only drawback is increasing latency for some requests, but it's rather simple to implement compared to introducing async. It is much harder to combine synchronous refresh on error and async approach, and using pure async approach will pass responsibility to implement non-trivial retry logic to app developers.

[Totktonada]

All requests that are guaranteed to write (insert, replace, delete,
upsert, update) use RW mode.

An RO instance can write to a replica local or a temporary space. Well, it is strange to write to a replica local space on some RO instance. However there may be a use case: say, register a task to be proceeded in background. I think we should have good defaults, but allow to choose anyway.

[Totktonada]

LGTM aside of several minor comments.

Please, finish review with Anastasia and proceed.

[AnaNek]

LGTM.