Fix sporadic unit test failure in rollback test

This issue was caused because pausing the flusher failed. The
reason was that we didn't wait for warmup and it was possible that
the command was sent before the flusher was even initialized.

Change-Id: I52da80c2d6d76f658f1525195ce521ac9dce55d5
Reviewed-on: http://review.couchbase.org/43076
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>