Fix ordering problem in connection_close for filesystem loaded in an extension

Consider a dynamically loaded library loaded via the extension mechanism that calls WT_CONNECTION->set_file_system. When the connection is closed, the wt_connection_close function first dlcloses the library, then calls wt_connection_destroy in order to call terminate on the file system. But the code to implement terminate has been unloaded in the dynamic library, the pc will point to unmapped memory.