purge_iter_t::operator<=(): Ordering comparison.
This replaces trx_purge_check_limit() with the difference that
we are not comparing undo_rseg_space. (In MariaDB, temporary
undo logs do not enter the purge subsystem at all.)

purge_sys_t::done: Remove. This was not used for anything.

purge_sys_t::tail: Renamed from purge_sys_t::iter.

purge_sys_t::head: Renamed from purge_sys_t::limit.