Session GC fix: save session.modified field as UTC so our comparisons work.

Session GC fix: save session.modified field as UTC so our comparisons work.

Had to tweak statusnet.ini to remove the DB_DATAOBJECT_MYSQLTIMESTAMP bitfield constant on session.modified; while it sounds like a useful and legit setting, it actually just means that DB_DataObject silently fails to pass through any attempts to explicitly set the value. As a result, MySQL does its default behavior which is to insert the current *LOCAL* time, which is useless.
This was leading to early GC west of GMT, or late GC east of it. Early GC could at worst destroy all live sessions (whoever's session *triggered* GC is fine, as the session then gets saved right back.)