Magma Encryption: Drop keys through compaction

CompactKVStore API should take a list of keys to be dropped.

Additionally, we can look to reuse purger code for this type of selective compaction. we can extract the predicate function that purger uses to pick a table for compaction. right now it is only cfg.NeedsPurging. we can make it based on keyID.

we have to make sure before purger runs, any in-flight flushes/compactions have finished (which may be using the keyID we're about to drop).