A session cookie now carries the id of a row in the new sessions table,
and is refused once that row is gone. :sessions lists the browsers
signed in, newest first, and :sessions rm <n> signs one out without
touching the rest; it locks the vault too, since the vault is open for
the whole workspace. Logging out deletes the row, :logout! and a login
password change delete them all, and the sweep drops expired rows.
last_seen is kept in memory and written at most every five minutes.
Cookies issued before this carry no id and are refused: every browser
logs in once more after the upgrade.
