Running a Purge and a Get at the same time could allow an old key to be
pulled back into the cache if the request was initiated by a node that
had been cleared and sent to a node that hadn't. This patch tries to
mitigate this, by adding an additional `generation` field, that keeps
track of the number of purges issued. Requests are fulfilled successfully
only if boths ends of a request are on the same generation.
* `Get()` now returns immediately when context is done during a groupcache peer
conversation. Previously `Get()` would call the `Getter` with a done context.
