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.
What's up with the alternative statement in the comment?