Convert memtable rotation to a channel message

[marvin-j97]

Currently the memtable rotation happens in the write hot path when the memtable size threshold is reached https://github.com/fjall-rs/fjall/blob/main/src/partition/mod.rs#L748-L749.
The performance hit mostly comes from the journal rotation which needs to fsync to disk.
It would be better for performance to send a message via channel, announcing that the partition needs to be flushed, nullifying rotation costs inside the write hot path, which should give quite a performance boost for write heavy applications (currently it is slightly slower than Rocks).
The messages should be picked up by a worker that rotates the partition asynchronously, essentially performing the Partition::rotate_memtable (https://github.com/fjall-rs/fjall/blob/main/src/partition/mod.rs#L633) function.