Introducing a new metric GetFromPeersSlowestDuration which will be recording the slowest
call made to getFromPeers.
Introducing SetLogger() to set a logger and allow getFromPeers error to be logged of.
* `Get()` now returns immediately when context is done during a groupcache peer
conversation. Previously `Get()` would call the `Getter` with a done context.
* Now Associating the transport with peer `httpGetter` so we take advantage of
connection reuse. This lowers the impact on DNS and improves preformance for
high request volume low latency applications.
* Now always populating the hotcache. A more complex algorithm is unnecessary
when the LRU cache will ensure the most used values remain in the cache. The
evict code ensures the hotcache does not over crowd the maincache.
requests could simultaneously result in a load(). Only requests that
enter singleflight Do concurrently would be deduped, so it was possible for
populateCache to be called multiple times for the same key. That
would result in overcounting nbytes, and eventually leading to a
livelock where nbytes > cacheBytes, but there were no entries in the
cache.
Change-Id: I5b304ce82041c1310c31b662486744e86509cc53
