TykTechnologies / tyk / 5129

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Do less session updates, remove hash check (#1770)

Previously, to know if the session needs to be updated or not, we had to generate some session hash (by serializing an object to msgpack), and if a hash of new session does not match a hash of original session, we triggered an update. This was causing unnecessary updates because change in any field changed hash, and hashing itself was quite slow. 

There are cases when fields changed in existing session object do not need to trigger updates. And this PR introduce the new way of manually specifying places when storage update is needed for the session or not. If you want update session, just call `ctxScheduleSessionUpdate(r)`, where `r` is current request object. It will set the request context boolean variable, and will schedule update, at the end of the request. As a big bonus, it will restrict running more than one session update per request.

The main showcases for this feature are applying policies and quota updates. 

In case of `ApplyPolicy`, we run this function every time, on each request, so session object always gets fresh data from policy, it does not really depend on data from original session object, except policy_id. Previously when policy got changed, it updated session, its `hash` changed as well, and it was triggering an update for each token. Now, `ApplyPolicies` never triggers session update. And keys when we need to see current key permissions with Tyk API, covered by running  `ApplyPolicies` one more time inside API call.

In case of quotas, on each request, we change `session.QuotaRemaining` field on each request, which triggers session update too. But in fact, `QuotaRemaining` always computed dynamically, and value from original session used only for Keys API, when you retrieve key and need to know current quota usage. Now, quota updates never trigger session update as well, which means minus one redis call in all cases when rate limiting used. Keys API now separately fe... (continued)

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