BlockchainDB implementations have names now

In order to make things more general, BlockchainDB now has get_db_name()
which should return a string with the "name" of that type of db.
This "name" will be the subfolder name that holds that db type's files
within the monero folder.

Small bugfix: blockchain_converter was not correctly appending this in
the prior hard-coded-string implementation of the subfolder data
directory concept.

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -775,6 +775,13 @@ std::vector<std::string> BlockchainLMDB::get_filenames() const
   return filenames;
 }
 
+std::string BlockchainLMDB::get_db_name() const
+{
+  LOG_PRINT_L3("BlockchainLMDB::" << __func__);
+
+  return std::string("lmdb");
+}
+
 // TODO: this?
 bool BlockchainLMDB::lock()
 {