db: dictionary compression

[jbowens]

Today Pebble's sstable compression is entirely per-block. If there's significant repetition across blocks within a sstable, the repetition is persisted. We could consider building dictionaries that are used across all the blocks of an sstable in the hopes of achieving better compression ratios. See RocksDB's feature:

See https://github.com/facebook/rocksdb/wiki/Dictionary-Compression