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/blockchain_db.h

@@ -326,7 +326,7 @@ public:
   void show_stats();
 
   // open the db at location <filename>, or create it if there isn't one.
-  virtual void open(const std::string& filename) = 0;
+  virtual void open(const std::string& filename, const int db_flags = 0) = 0;
 
   // make sure implementation has a create function as well
   virtual void create(const std::string& filename) = 0;