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:

fjall/src/partition/mod.rs

Lines 747 to 752 in 9395ba1

	pub(crate) fn check_memtable_overflow(&self, size: u32) -> crate::Result<()> {
	if size > self.config.max_memtable_size {
	self.rotate_memtable()?;
	self.check_journal_size();
	self.check_write_halt();
	}

The performance hit mostly comes from the journal rotation which needs to fsync 2x (?) 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 function:

fjall/src/partition/mod.rs

Line 633 in 9395ba1

pub fn rotate_memtable(&self) -> crate::Result<bool> {