Do not write to the cache if the plugin decides not to write

[vmamidi]

TSHttpTxnServerRespNoStoreSet can be called after READ_REQUEST_HDR state. For example, cache promotes plugin calls this at TS_EVENT_HTTP_CACHE_LOOKUP_COMPLETE hook.
Checking the api_server_response_no_store flag after receiving the origin server response could create a race condition when a request that is not chosen to update the cache by cache promote plugin is holding the lock.

request1---holds the locks( not chosen to write to cache)---request2---fails to acquire the lock(selected to write to cache)

This causes both the requests to not write to cache.

Checking for the flag api_server_response_no_store before moving into the CACHE_ISSUE_WRITE state avoids the cache write lock and avoids the race.