lei import|lcat: improve+fix single message IMAP support
lcat can now dump the memoized contents of entire IMAP folders,
not just a single UID. It's now parallelized and pipelined for
multiple lei2mail workers.
Furthemore, various forms of JSON output work consistently
with blob-only output, now.
While working on this, I noticed NetReader was passing UID URLs
to imap_each callbacks, which was causing mail_sync.sqlite3 to
store UIDs in `folders' and clearly wrong so it's now fixed.