Use cb_assert instead of assert for windows compatibility

Change-Id: I1c2c326f60dd16c595aee22856f53f5b78553e55
Reviewed-on: http://review.couchbase.org/38155
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>