RocksDB: VBState is persisted out-of-order with respect to updates

In saveDocs, we prepare a batch of updates to write to the DB, followed by a persisted vbState, and then the batch is committed. However the saveVBState is done using a non-batch Put(), which means that the VBstate is persisted before the updates.

This is incorrect, as in the event of a crash we would see the wrong high seqno etc in the vbState compared to the data.