Add support for database open with flags

Add support to:
  - BlockchainDB, BlockchainLMDB
  - blockchain_import utility to open LMDB database with one or more
    LMDB flags.

Sample use:
  $ blockchain_import --database lmdb#nosync
  $ blockchain_import --database lmdb#nosync,nometasync

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -635,9 +635,8 @@ BlockchainLMDB::BlockchainLMDB(bool batch_transactions)
   m_height = 0;
 }
 
-void BlockchainLMDB::open(const std::string& filename)
+void BlockchainLMDB::open(const std::string& filename, const int mdb_flags)
 {
-  int mdb_flags = 0;
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
 
   if (m_open)