Memstore scanner needs new semantics, which may require new data structure

We may need a new memstore datastructure. Much has been written about the concurrency and speed and cpu usage, but there are new things that were brought to light with HBASE-2856.

Specifically we need a memstore scanner that serves up to the moment reads, with a row-level completeness. Specifically after a memstore scanner goes past the end of a row, it should return some kind of 'end of row' token which the StoreScanner should trigger on to know it's at the end of the row. The next call to memstore scanner.next() should return the very next available row from the start of that row at the time it's requested.

It should specifically NOT:

• return everything but the first column
• skip a row that was inserted after the previous next() was completed