Slow down Kombu reconnect attempts

For a rationale for this patch, see the discussion surrounding Bug

When reconnecting to a RabbitMQ cluster with mirrored queues in
use, the attempt to release the connection can hang "indefinitely"
somewhere deep down in Kombu.  Blocking the thread for a bit
prior to release seems to kludge around the problem where it is
otherwise reproduceable.

DocImpact

Change-Id: Ic2ede3046709b831adf8204e4c909c589c1786c4
Partial-Bug: #856764
Merge "Slow down Kombu reconnect attempts"