Create MySQL store and refactored H2 & Postgres stores

1) Refactored PostgresFullPrunedBlockStore and H2FullPrunedBlockStore into a generic DatabaseFullPrunedBlockStore class.
2) H2FullPrunedBlockStore now supports calculateBalanceForAddress() method.
3) Created an additional store for MySQL (MySQLFullPrunedBlockStore) and unit test.
4) Fixed unit test PostgresFullPrunedBlockStoreTest, second time execution was failing due to not clearing down the tables correctly.
5) Added methods getOpenTransactionOutputs() & deleteStore() to base class DatabaseFullPrunedBlockStore.
Ok this wasn't a method I added (in fact all the clever stuff is simply cut and paste from the previous code base :). I did notice that it returned BigInteger but I left it like that as this will would break the API (not backward compatible).  I did think at the time that this was probably just missed when the Coin class was introduced.  Shall I change it still?