core: disable rw triggers on replicas

[ShelbyKS]

No description provided.

[Col-Waltz]

Thank you for the patch!

Please provide proper commit message for your changes:

core: disable rw triggers on replicas
<Some information about changes and implementation reason>

[ShelbyKS]

provided propper commit message

[Col-Waltz]

Thanks for providing a proper commit message, now I see some problems in this solution:

1. ERR_READONLY error signals that we try to modify data on RO instance. It is more safe to rely on box.info.ro to protect RO instanse from modifying.

2. _on_consumer_disconnect wakes all waiters so they can close their connections. Waiters operate with local fields and do not modify data itself, but it is useful if this instance will become master again. So it is safe to wake them up and it is better to do this so that they don't wait ttl.

I suggest to fix _on_consumer_disconnect like this:

local consumers = box.space._queue_consumers
local waiters = consumers.index.pk:select{conn_id}
for _, waiter in ipairs(waiters) do
local fid = waiter[2]
local cond = conds[fid]
if cond then
releasing_connections[fid] = true
cond:signal(fid)
end
if not box.info.ro then
consumers:delete{waiter[1], waiter[2]}
end
end

So we will wake up waiters, but delete them only if it is safe.

And session_disconnect also should be protected:

if not box.info.ro then
session.disconnect(conn_id)
end

I will implement this changes myself in other PR.

[Col-Waltz]

Implemented via #252