Proposal: Allowing WriteBatch to emulate a memtable

### Expected behavior
When we creating a large `WriteBatch`, this WriteBatch can be Wrapped as a memtable.

### Actual behavior
`WriteBatch` is always write to memtable, regardless how large it is. This waste memory and drop down performance.

### Steps to reproduce the behavior
Put 1GB data into a single WriteBatch.

### Further more ( And proposed implementation ... )
1. This trick can also be applied on `WriteBatchWithIndex` to improve MyRocks performance.
1. This **Fake** memtable need some kind of index to WriteBatch's internal data.
1. For multiple ColumnFamily, this trick also works, we just need a shared_ptr on each memtable, let it point to WriteBatch internal data.