MB-5010 Reset the thread local data whenever leaving a bucket.

A thread local data is set to a bucket pointer when a given
thread enters into that bucket for supporting the memory
accounting per bucket. This means that the thread local data
should be reset when the thread leaves from its bucket.

Conflicts:

        ep_engine.cc

Change-Id: I02124045c4503bf5d860e1063ab4d0aa11fd43b1
Reviewed-on: http://review.couchbase.org/14773
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-on: http://review.couchbase.org/14919