This commit introduces EXP_Reduce(), a function to reduce object timers.
The goal is to provide a function better suited to soft-purging objects,
as EXP_Rearm() has some non-obvious disadvantages when used or this
purpose.

When EXP_Rearm() is used to soft-purge an object by setting its TTL to
0, the expiry is effectively reset to the start of the objects grace
period. This happens because the object TTL includes a time delta
between object insertion time (oc->t_origin) and now. The result is that
a soft-purge extends the lifetime of an already stale object, and
repeated soft-purges can keep the object away from expiry indefinitely.

The EXP_Reduce() function is better suited for soft-purging, as it only
updates an objects TTL if it would reduce the objects lifetime. The
function also has facilities for reducing grace and keep.