lei_store: keywords => vmd (volatile metadata), prepare for labels
Since keywords and mailboxes (AKA labels) are separate things in
JMAP; and only keywords can map reliably to Maildir and mbox;
we'll keep them separate in our internal data representations,
too.
I initially wanted to call this just "meta" for "metadata", but
that might be confused with our mailing list name. "metadata"
is already used in Xapian's own API, to add another layer of
confusion.
"tags" was also considered, but probably confusing to notmuch
users since our "labels" are analogous to "tags" in notmuch,
and notmuch doesn't seem to cover "keywords" separately...
So "vmd" it is, since we haven't used this particular
three-letter-abbreviation anywhere before; and "volatile" seems
like a good description of this metadata since everything else
up to this point has been mostly WORM (write-once, read-many).