session-per-request to session-over-time

[runawaycoast]

this PR changed from session-per-request to session-over-time.

previous implementation session-per-request:
Every time we call API with get*, it will create a session within get_session() function.

Current implementation session-over-time:

1. System creates a global variable session, initially be assigned with None.
2. Once get_session() has been called for the first time, it will create the global session and setup retry strategy for it. (*)

• Retry strategy can only be setup on session level. If we create session in global, the code will not respect the APIConfig retry strategy (ApiConfig could be designed to be a config object to pass into each request, instead of make functions depend on a global one)

---

con:

• clients cannot change the retry strategy after the first request, (System can add a listener on ApiConfig, and every time clients change retry config, system generate a new session)

	1	5	50	100	500	1000
non-reuse - function count	357108	398632	865777	1384827	5537227	10727727
reuse - function count	357119	396816	843351	1339501	5308701	10270201
	1	5	50	100	500	1000
non-reuse - second	0.55	0.69	1.42	2.43	8.71	16.768
reuse - second	0.53	0.61	0.9	1.38	4.67	8.68

[runawaycoast]

Close this pr in favour of #22