lei import: keep sync info for Maildir and IMAP folders
We aren't using it, yet, but the plan is to be able to use
this information to propagate keyword changes back to IMAP
and Maildir folders using some to-be-implemented command.
"lei inspect" is a half-baked new command to make testing this
change easier. It will be updated to support more SQLite+Xapian
introspection duties in the future, including public-inbox
things independent of lei.