Compaction filters

[iczero]

• Introduce CompactionFilter trait which can elect to drop a kv or modify its value
• Introduce ItemAccessor for CompactionFilter which can access key/value within the compaction filter
• Rename ExpiredKvCallback to DroppedKvCallback since it's not really exclusively for "expired" anymore
• Add filter support to CompactionStream

Compaction filters replace dropped KVs with a tombstone. If we are compacting to the last level, the tombstone will subsequently be removed by the CompactionStream. If the value is changed, it is replaced in place or written out to a new blob file as necessary.

Closes #159

[codecov]

Codecov Report

❌ Patch coverage is 97.76536% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/compaction/filter.rs	95.87%	4 Missing ⚠️

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[iczero]

I've added a finish method to CompactionFilter as well. It's possible to accomplish much of the same using Drop, but finish takes ownership of self and as such is a tad bit more flexible.

[marvin-j97]

I've been thinking of calling this "compaction mappers" instead of "compaction filters" because "filters" is already quite a loaded term (AMQ filters and filter() in Rust), while this operation is more akin to a filter_map really.

[iczero]

In my opinion, "filter" would probably fit better in this case, but I'm fine with either name. Should I change it?

[marvin-j97]

In my opinion, "filter" would probably fit better in this case, but I'm fine with either name. Should I change it?

I think it's fine either way