Pool memcache connections

This creates a pool to each memcache server so that connections will not
grow without bound.  This also adds a proxy config
"max_memcache_connections" which can control how many connections are
available in the pool.

A side effect of the change is that we had to change the memcache calls
that used noreply, and instead wait for the result of the request.
Leaving with noreply could cause a race condition (specifically in
account auto create), due to one request calling `memcache.del(key)` and
then `memcache.get(key)` with a different pooled connection.  If the
delete didn't complete fast enough, the get would return the old value
before it was deleted, and thus believe that the account was not
autocreated.

ClaysMindExploded
DocImpact
Change-Id: I350720b7bba29e1453894d3d4105ac1ea232595b
Merge "Pool memcache connections"