LMDB blockchain: remove outputs and spent keys
All LMDB BlockchainDB implemented, not tested

All of the functionality for the LMDB implementation of BlockchainDB is
implemented, but only what is in tests/unit_tests/BlockchainDB.cpp has
been tested.  This is basically add a block, see if you can get the
block and a tx from the block.  More tests should be added at some
point.