Move backend dictionary and state to MdbHandle

Attempt to make the backend handling logic thread-safe. This removes the
last MDB_CONSTRUCTOR. Also get rid of some JAVA junk and make the
remaining static variables in backend.c constant. Finally remove some
obsolete fields from MdbFile.

src/libmdb/file.c

@@ -307,6 +307,7 @@ mdb_close(MdbHandle *mdb)
 	}
 
 	mdb_iconv_close(mdb);
+    mdb_remove_backends(mdb);
 
 	g_free(mdb);
 }