lei import: avoid IMAPTracker, use LeiMailSync more
IMAPTracker has a UNIQUE constraint on the `url' column,
which may cause compatibility and/or rollback problems
in attempting to deal with UIDVALIDITY changes.
Having multiple sources of truth leads to confusion and bugs,
so relying on LeiMailSync exclusively ought to simplify things.
Furthermore, since LeiMailSync is only written to by LeiStore,
it is safer in that it won't mark a UID or article as imported
until git-fast-import has seen it, and the SQLite commit always
happens after "done\n" is sent to fast-import.
This mostly reverts recent commits to IMAPTracker to support
lei, those are:
1) commit
7632d8f7590daf70c65d4270e750c36552fa9389
("net_reader: restart on first UID when UIDVALIDITY changes")
2) commit
311a5d37ad275cd75b1e64d87827c4d13fe4bfab
("imap_tracker: prepare for use with lei").
This means public-inbox-watch will not change between 1.6 and
1.7: -watch stops synching a folder when UIDVALIDITY changes.