Replicator batch

This adds the ability to store changes inside the same database, in a sublevel. Both changes and their logs commit to the database in the same batch, so there is no way for data and metadata to get out of sync.

To use a sublevel rather than a separate database, pass the string "sublevel" to the replication server.

    replicator.install(my_db, "sublevel", serverConfig);

This change is 100% backwards-compatible. Either the server or client (or both, or neither) can use the sublevel rather than the separate database.