add write_through option to disk_io_write_mode, which will attempt to flush pieces to disk once they're validated (with msync). Also make disable_os_cache also flush write blocks and mark all cache blocks as cold

bindings/python/src/session_settings.cpp

@@ -39,6 +39,7 @@ void bind_session_settings()
         .value("disable_os_cache_for_aligned_files", settings_pack::disable_os_cache_for_aligned_files)
 #endif
         .value("disable_os_cache", settings_pack::disable_os_cache)
+        .value("write_through", settings_pack::write_through)
     ;
 
     enum_<settings_pack::bandwidth_mixed_algo_t>("bandwidth_mixed_algo_t")