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.
hmm It's also a public part of the API.

public BigInteger calculateBalanceForAddress(Address address) throws BlockStoreException {
        return calculateBalanceForAddress(address, false);
    }

The thing is this will effect people that are currently using the Postgres store....