largefiles: makes verify batching stat calls to remote Instead of sending stat calls for each files separately, it sends one batch call with stat invocations for all files.

largefiles: change basestore._verifyfile to take list of files to check Makes it easier to use batch stat calls in remotestore to decrease number of round trips.

bookmarks: properly invalidate volatile sets when writing bookmarks This corrects a regression introduced during the 3.7 cycle, but which went undetected due to the surviving-but-deprecated write() method on bmstore.