MB-11347 Set thread local to NULL before calling get_failover_log

get_failover_log API is the callback function provided by
memcached layer and internally allocates more memory for buffer,
which will be freed later. Therefore, it shouln't be accounted
for a bucket memory usage.

Change-Id: I23324f88e6c87e9cb6e3d067cef2f2f2efffbf04
Reviewed-on: http://review.couchbase.org/38916
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>