Revert "Allow Java byte arrays to be directly encoded for byte sequence types."

This reverts commit d2ab4f67701a5bd911577e26bfd4bcf98d9ff1e3.

This undoes https://github.com/seomoz/askari/pull/69
Don't encode java byte arrays.

Assume they're already encoded instead.
`Relation#do_scan` returns a `#rows` enumerator.

Scans no longer flatten their result sets into a single stream of
`KeyValues`.  Instead they respect row boundaries by using `#rows`, a
new method that enumerates rows (which are `#key_values` enumerators.)

KeyValue enumerators now include a `#cells`, which curries themselves
into `Relation#cells`, allowing them to convert into cell enumerators.

The typical scanning use case now requires users to enumerate over
rows, and if decoding is desired to call `#cells` on each row
enumerator.
`Relation#do_get` returns a `KeyValue` enumerator.

To get cells, call `#cells` on the enumerator itself.