MB-100: _unlocked_update hangs with [Errno 32] Broken pipe

http://qa.hq.northscale.net/view/2.0.1/job/ubuntu-64-2.0-new-rebalance-tests-P0/329/console

Change-Id: Ifc1dd54381caf28e5e32afcda0bf2f2c2f4bce98
Reviewed-on: http://review.couchbase.org/24127
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.salooja@globallogic.com>