spacemanager: Replace lifetime column with expirationtime column

Uses a null value to represent that there is no expiration time (infinite
lifetime). Also removes indexes on creationtime columns.

Storing expiration time is much more useful as that's what we typically
search for (has expiration time passed).

Changes the DAO API to simplify updateSpace and thus also enable it to
store null.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6483/