IMAP connection management.

Connect to an IMAP server and obtain a store that can be passed
to functions in `mailseq.imap.folder` and `mailseq.imap.fetch`.

Fetch IMAP messages by date range and/or by absolute count.

mailseq is read-only and driven by just two selection criteria:
`:since`/`:before` (via `SentDateTerm`) and `:limit` (via UID-range
optimisation or tail-of-result). The IMAP backend does not use
`SEARCH FROM/TO/SUBJECT/…` because those verbs are implemented
inconsistently across servers.

All functions return parsed Clojure maps (via
`mailseq.parse/message->map`).

IMAP folder operations: list, open, close, and query folders.

IMAP IDLE support for receiving push notifications when new messages arrive.

IDLE keeps a connection open and the server notifies the client when
the folder state changes (new messages, deletions, flag changes).

A heartbeat thread periodically breaks out of IDLE to force a NOOP
roundtrip, preventing NATs/firewalls/servers from killing the connection.