merge: factor Context out of mailbox functions

merge: factor Context out of mailbox functions

 * imap: drop Context from imap_copy_messages()
 * tidy mbox_close
 * mailbox: add notify callback
 * mx_fastclose_mailbox: use mailbox notifications
 * mx_fastclose_mailbox: factor out Context
 * imap_expunge_mailbox: use mailbox notifications
 * imap: drop mdata->ctx
 * mbox_open: factor out Context
 * mbox_check: use mailbox notifications
 * mbox_check: factor out Context
 * mbox_sync: use mailbox notifications
 * mbox_sync: factor out Context
 * split out context functions
 * mx_mbox_close drop index_hint
 * imap tidy