Add a Timeout when getting Memcache connections

The old Timeout behavior when pulling connections of the MemcacheConnPool left
ambiguity around what timed out and could put more placeholders in the queue
than the configured max_connections.

To avoid waiting indefinitely on slow severs we raise a custom Timeout when we
fail get a connection from the pool.  We still error limit the slow server,
and move onto the next, but we still don't allow more than max_connections.

Change-Id: I9e2409896423d52da69e35c038e5f457c71f705d
CHANGELOG and AUTHORS updates for 1.10.0 release

Change-Id: Iaf8d289c5ddb6ac778243a5e1fc27600bade6feb
Merge "Add a Timeout when getting Memcache connections"
Merge "CHANGELOG and AUTHORS updates for 1.10.0 release"