mq: only save dirty files once when savedirty is called multiple times

mq: remove early wlock release in qnew It seems like something that missed the refactoring in 97b734fb9c6f.

mq: use .invalidate to cancel dirty mq state when cancelling transaction Before the code optimistically relied on savedirty not being called a cancelled transaction. If it was called it could save incorrect data. Instead we now start using the invalidate method introduced in b169ba60eebe.