lib-storage: Added mailbox_recent_flags_expunge_uid() For storage backends where it's useful.

lib-storage: Added mailbox_recent_flags_set_uid_forced() This allows adding recent flags to mails whose UID is lower than the existing recent mails.

lib-storage: Moved most of the \Recent flag handling code to mailbox-recent-flags.c There are also some API changes, because functions were renamed and the recent_* fields were moved to struct mailbox. I'm not aware of any plugins using these though, except for index_mailbox_set_recent_seq() which for now is kept as a backwards compatibility macro. No changes were made to the actual code logic.