MB-8214 & MB-8323 allocate iomanager threads per bucket

Based on the latest litmus tests still the 2:1 ratio of
readers and writers threads renders the best result
without previously measured get perfroamce regression.

This commit include changes that allocate ExecutorThreads
per bucket while still maintaning single global IOManager
instance. Also these changes allow a better way of
handling of thread termination after the bucket shutdown,
which in turn eliminates the crash observed in MB-8323.

Change-Id: I01d045277911cc461f04ef34df5251ccfb397124
Reviewed-on: http://review.couchbase.org/26560
Tested-by: Jin Lim <jin@couchbase.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>