Major compaction API

[marvin-j97]

This is already partially implemented as a hidden function, however it needs to be exposed as a method in AbstractTree (so it can be called for BlobTree as well, which will require a full rewrite of the entire value log (...maybe not)).

lsm-tree/src/tree/mod.rs

Lines 462 to 478 in 3a2e3e1

	// TODO: Expose as public function, however:
	// TODO: Right now this is somewhat unsafe to expose as
	// major compaction needs ALL segments, right now it just takes as many
	// as it can, which may make the LSM inconsistent.
	// TODO: There should also be a function to partially compact levels and individual segments
	/// Performs major compaction, blocking the caller until it's done.
	///
	/// # Errors
	///
	/// Will return `Err` if an IO error occurs.
	#[doc(hidden)]
	pub fn major_compact(&self, target_size: u64, seqno_threshold: SeqNo) -> crate::Result<()> {
	log::info!("Starting major compaction");
	let strategy = Arc::new(crate::compaction::major::Strategy::new(target_size));
	self.compact(strategy, seqno_threshold)
	}

It's important that the major compaction can get all segments, so it needs a write lock and wait until it can hide all segments.