Phs/fast bypass byte arrays in encoding

Create a fast-path in the client for moving byte arrays around without decoding.  A handful of changes to this effect:
 * `BaseType#encode` bypasses (immediately returns) literal java byte arrays, assuming they're already encoded.
 * `Relation#do_get` returns a `KeyValue` enumerator, instead of decoding the cells off the bat.
 * To compensate, key value enumerators now have a `#cells` to opt into cell decoding.
 * `Relation#do_scan` no longer flattens rows.  Instead it enumerates over row enumerators.  A row enumerator enumerates over `KeyValue`s, which can be decoded with the same `#cells`.

Reverts https://github.com/seomoz/askari/pull/69