memdb: compact the values left in mostly dead blocks

[mumtaz6]

Adds memdb.DB.Compact, which frees the WAL logs that long-kept values hold, and runs it in the server's message store.

The problem

• Why logs stay: a block's logs stay in the WAL while it holds a value, and so do the logs of every block that deletes from it, transitively. A delete must outlive the put it deletes, or the deleted value comes back after a restart.
• The effect: in a store that rewrites and deletes its keys, one long-kept value (a session row, or a message waiting on an absent subscriber) holds the logs of most blocks written after it.
• Seen in practice: a server's message store on v0.7.0 kept every log since it started, about 12 a minute while idle. Another held 1,379 logs for 1,644 keys, and every one of its 558 blocks was held this way.

Compact

• What moves: values in past blocks where live values take up at most half the block's data, which moves only what was left behind. Those blocks then hold nothing and are freed, with the logs that waited on them. Mostly live blocks stay.
• No write pause: each value moves under its key's index lock, as a Put does. A put or delete of that key waits; other writes carry on. A key put again or deleted since it was listed doesn't move.
• Opt-in: the engine's own memdb doesn't compact, because the engine finds entries by their block.
• Counters: Varz reports compactions and values moved.

The server's message store compacts at open and every minute.

Tests

• Pinned blocks: a key kept while others were rewritten and deleted held 152 logs. Compact moved 2 values and left 1 log; all values read correctly after a reopen.
• Mostly live blocks: nothing moves.
• Concurrent writes: about 95 compactions overlap writers putting and deleting their own keys. Each key keeps its last value, before and after a reopen.
• Model test: Compact is now one of its operations.
• Server adapter: four seconds of churn with a kept message leave 1 log with the compactor running, and 267 without it.
• Results: memdb, its model test (100 seeds) and memdb under -race -tags lockcheck pass; so do the engine, the server under -race, and e2e.

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