Fix race condition in test 18-monitor-partition-updates.t

Ensure that the updater is not triggered automatically when
there are blocked clients who are monitoring partition updates.
This accomplished by making the gen_server timeout configurable
for unit tests only.

Change-Id: I8e89e9f041303ef233d445c080e97c40b4290152
Reviewed-on: http://review.couchbase.org/26264
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>