These commands will be used by strongly consistent tablets to submit
mutations to Raft. A simple state_machine implementation is introduced
to apply these commands.
We apply commands in batches to reduce commitlog I/O overhead. The
batched variant of database::apply has known atomicity issues. For
example, it does not guarantee atomicity under memory pressure: some
mutations may be published to the memtable while others are blocked in
run_when_memory_available. We will address these issues later.
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