support snapshot policy where flush memstore can be skipped to prevent production cluster freeze

We are using snapshot feature to do HBase disaster recovery. We will do snapshot in our production cluster periodically. The current flush snapshot policy require all regions of the table to coordinate to prevent write and do flush at the same time. Since we use WALPlayer to complete the data that is not in the snapshot HFile, we don't need the snapshot to do coordinated flush. The snapshot just recored all the HFile that are already there.

I added the parameter in the HBase shell. So people can choose to use the NoFlush snapshot when they need, like below. Otherwise, the default flush snpahot support is not impacted.

>snaphot 'TestTable', 'TestSnapshot', 'skipFlush'