Pass parsed user key to prefix extractor in V2 compaction

Previously, the prefix extractor was being supplied with the RocksDB
key instead of a parsed user key. This makes correct interpretation
by calling application fragile or impossible.
Update HISTORY.md
Merge pull request #230 from cockroachdb/spencerkimball/send-user-keys-to-v2-filter

Pass parsed user key to prefix extractor in V2 compaction