While the LRU package has the ability to purge all items from cache,
this functionality was not available to `ProtoGetter`, making it
imposibile to clear the cache without restarting all peers. This change
adds a `Clear()` method to `ProtoGetter`, that enables clearing the
cache with no downtime.
* `Get()` now returns immediately when context is done during a groupcache peer
conversation. Previously `Get()` would call the `Getter` with a done context.
This change caused a data race during async calls to groupcache.Get().
Also discovered `DefaultTransport` has per address connection pooling
only when the request was a success, which is sufficient for most use
cases.
* 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.
