MB-100: _check_valid_keys hangs due to TimeoutError

as example http://qa.hq.northscale.net/view/2.0.1/job/centos-64-2.0-new-rebalance-mixed-cluster/38/consoleFull

Change-Id: I2ee19d70569cabbf9ba9946fd62028563487484e
Reviewed-on: http://review.couchbase.org/24740
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>