MB-11505 upr client: Add time out log message for vbucket stats

Added a log message if any of the vbucket stats requests take
more than 2 seconds for receiving the response. Stats request
will also block wait until response is received or the upr client
crashes due to any internal error.

Change-Id: I76992f198b7b12e758aa50a8c43b4e37d215c29f
Reviewed-on: http://review.couchbase.org/39086
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>