Transaction DB holds too much memory

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We have a scenario needs multiple DB instances and also multiple CFs within a DB with transaction support.
Transaction DB keeps **max_write_buffer_number** of memtable even when it is flushed, considering the number of DB instance times the CF count, it's significant of memory consumption.
I know those memtable is held for checking write conflicts, but it doesn't seem to be freed when there is no write for a while.
The work around for us is setting **max_write_buffer_number_to_maintain = 1,** but even so, it still uses a lot of memory.

### Expected behavior
Tx DB should free memtable when there is no write for a while, or at least can be freed by manual flush, so the user can decide to free up the memory when there is no recent write.

### Actual behavior
Tx DB consistently holds max_write_buffer_number_to_maintain of memtables, even there is no recent write. Manual flush doesn't help either.

### Steps to reproduce the behavior

Create TransactionDB with max_write_buffer_number = 3, write_buffer_size=64M. Then write enough data (more than 3 * 64MB). DB holds ~192MB memory forever, even after manual flush. If there is N CF in the DB, then the memory overhead becomes even more significant.

Cc: @yuslepukhin