Rust: example project compacts entire database every compaction

[brson]

This is an obvious but understandable weakness, particularly while compaction happens non-concurrently.

Need to see how the text describing the log files corresponds to the example. How open the text leaves the compaction strategy, or whether we are speccing that the entire DB is compacted at once.

Project 4 has a yet-unwritten extension to do compaction concurrently on its own thread. If that can be done without interrupting the writer thread, that would definitely reduce the negative effects of huge compactions.

A good future project would be to figure out how to spread out compaction so it doesn't all happen at once.