Make expiry time for paged models settable from props

Make expiry time for paged models settable from props

Our paged models used the default minimumExpiryTime of zero seconds
which caused frequent stutter with AI traffic loaded.
This patch sets the minimumExpiryTime to 180 seconds, more than enough
to hold the models in memory during a full standard rate turn.

The property to set the expiry time is
/sim/rendering/plod-minimum-expiry-time-secs