MB-11347 Set thread local to NULL before calling upr_producers API

As upr_message_producers APIs provided by memcached layer can
allocate additional memory that will be freed later, we should
set the thread local to NULL before calling those APIs, so that
the allocated memory inside those APIs shouldn't be accounted for
a given bucket instance.

Change-Id: I31eac3fdc505fbde32a9b54ac8914d9d56e8992a
Reviewed-on: http://review.couchbase.org/38863
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>