db_lmdb: warn about slowness when running off a spinning disk

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -1178,6 +1178,9 @@ void BlockchainLMDB::open(const std::string& filename, const int db_flags)
     throw DB_ERROR("Database could not be opened");
   }
 
+  if (tools::is_hdd(filename.c_str()))
+    MCLOG_RED(el::Level::Warning, "global", "The blockchain is on a rotating drive: this will be very slow, use a SSD if possible");
+
   m_folder = filename;
 
 #ifdef __OpenBSD__