Add row-level isolation

This could be done using an the atomic ConcurrentMap operations from the Memtable and something like http://code.google.com/p/pcollections/ to replace the ConcurrentSkipListMap in ThreadSafeSortedColumns. The trick is that pcollections does not provide a SortedMap, so we probably need to write our own.

Googling [persistent sortedmap] I found http://code.google.com/p/actord/source/browse/trunk/actord/src/main/scala/ff/collection (in scala) and http://clojure.org/data_structures#Data Structures-Maps.