Bug 31034

From #13847:

I've looked at the TimedSoftRef. Functionally is
it OK,
but I'd suggest using a bit smarter scheme for
timeout impl:
The (re)schedule call is relatively expensive, it
does some
synchronization, allocates some objects and, worst
of all,
each schedule(time!=0) leaves a trail in the
TimerTask, a runnable
that will be executed (and possibly do nothing).
So the current impl would behave poorly for
sequences of get().

I suggest using this approach:
On first get() schedule a task (now+30000) and
record the
time-to-clear (now+30000). For subsequent get()s
(TTC is nonzero),
just update the TTC to now+30000.
In the timeout handler, compare "now" to TTC and
if it is bigger,
clear the reference and clear the TTC, otherwise
reschedule the task
using schedule(TTC-now).
That means at most one reschedule each 30000ms per
reference.

Maybe it can be implemented directly in the RPs
reschedule,
but this way it it quite isolated and simple code.