query: perform read repair

[peterbourgon]

Queries can perform read repair, by counting each unique ULID (and thus log record) encountered during the deduplicate phase. Those which have lower representation than expected can be bundled together into a repair segment, which can be replicated through the normal replication mechanisms. Care should be taken to only do this work some percent of all queries; to perform the replication outside of the query lifecycle; etc.