Add first AsyncTable impl and create TableImpl based on it

First we create an AsyncTable interface with implementation without the Scan functionality. Those will land in a separate patch since they need a refactor of existing scans.

Also added is a new TableImpl to replace HTable. It uses the AsyncTableImpl internally and should be a bit faster because it does jump through less hoops to do ProtoBuf transportation. This way we can run all existing tests on the AsyncTableImpl to guarantee its quality.