Fix race condition when timing out an operation using ExecuteRequest
Fix race condition when timing out an operation using ExecuteRequest

If the timeout occurs before we call the callback and the callback
is called then we will block waiting for someone to read the channel.
The common go solution for this is to allow 1 item to enter the
channel before blocking.

Change-Id: I7e4120e78229e013516861718854cd9b020cee05
Reviewed-on: http://review.couchbase.org/57846
Well-Formed: buildbot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Tested-by: Michael Wiederhold <a17fed27eaa842282862ff7c1b9c8395a26ac320@couchbase.com>
Reviewed-by: Brett Lawson <c8c4369773d7d7226e031f0c4af62e3f38a08510@gmail.com>

brett19/gocb