Steps toward multiple dbs available -- working

There will need to be some more refactoring for these changes to be
considered complete/correct, but for now it's working.

new daemon cli argument "--db-type", works for LMDB and BerkeleyDB.

A good deal of refactoring is also present in this commit, namely
Blockchain no longer instantiates BlockchainDB, but rather is passed a
pointer to an already-instantiated BlockchainDB on init().

src/blockchain_db/blockchain_db.cpp

@@ -129,6 +129,11 @@ void BlockchainDB::pop_block(block& blk, std::vector<transaction>& txs)
   }
 }
 
+bool BlockchainDB::is_open()
+{
+  return m_open;
+}
+
 void BlockchainDB::remove_transaction(const crypto::hash& tx_hash)
 {
   transaction tx = get_tx(tx_hash);