Support KeyValueCodec to encode non KeyValue cells.

We have KeyValueCodec as the default RPC codec now. This can only encode KeyValue types and need to convert any non KV cell to KeyValue format. This is a expensive op.

  // This is crass and will not work when KV changes. Also if passed a non-kv Cell, it will
  // make expensive copy.
  // Do not write tags over RPC
  KeyValue.oswrite((KeyValue) KeyValueUtil.ensureKeyValue(cell), this.out, false);

Now we want Cell to be in entire read path and don't want any copy until we write the cell to client.
The optimization done in the DBE of avoiding copying of value bytes was also not getting real advantage bacause of this ensureKeyValue() call at the Codec.

We can encode and write non KeyValue cells with KV serialization format and avoid the recreate.