Basic Block Reference Counting

This patch adds basic block reference counting to all metadata drivers. Block counts are incremented and decremented accordingly. A future patch will hook this up to enforce the inability to delete blocks when they have references. 
@BenjamenMeyer, this is the sqlite driver, not the cassandra driver. 
@TheSriram, every row in this table is a reference to a particular block so counting all of the rows where a particular block occurs gives you to total number of references to that block. Does that answer the question?