db_lmdb: don't sync a read only DB

This would only throw

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -1391,6 +1391,9 @@ void BlockchainLMDB::sync()
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);
   check_open();
 
+  if (is_read_only())
+    return;
+
   // Does nothing unless LMDB environment was opened with MDB_NOSYNC or in part
   // MDB_NOMETASYNC. Force flush to be synchronous.
   if (auto result = mdb_env_sync(m_env, true))