Enhance forced eviction to allow multiple pages to be queued.

This implementation creates a queue of forced pages during the eviction pass (but shares the regular eviction queue).

It'd be possible to do the following instead:
* Section off a part of the regular eviction queue for forced candidates
* Create a separate queue for forced candidates

Given that having pages queued for forced eviction is relatively infrequent, allocating a queue (array) when we have forced eviction candidates and the eviction server isn't regularly running seems like a reasonable compromise for keeping the code a bit simpler.

Refs issue #491