lei_store: local storage for Local Email Interface
Still unstable, this builds off the equally unstable extindex :P
This will be used for caching/memoization of traditional mail
stores (IMAP, Maildir, etc) while providing indexing via Xapian,
along with compression, and checksumming from git.
Most notably, this adds the ability to add/remove per-message
keywords (draft, seen, flagged, answered) as described in the
JMAP specification (RFC 8621 section 4.1.1).
We'll use `.' (a single period) as an $eidx_key since it's an
invalid {inboxdir} or {newsgroup} name.