lib-storage: Don't reset mail_save_context.saving too early.

lib-storage: Don't reset mail_save_context.saving too early.

If mailbox_save_using_mail() ended up in mail_storage_copy(), saving was set
to FALSE before the copy() method was finished running. This caused e.g.
notify plugin to think that this was a copy event instead of a save event.

Added comments and asserts to clarify how the logic should work between
all the different copying/moving/saving flags.