Eviction under load

While looking at #1068, one thing I noticed is that the eviction server thread can get "stuck" in eviction (particularly splits), and that other threads stall.  This branch attempts to address that in two ways:

1. if other threads are waiting to do eviction, the server thread just wakes them up and keeps scanning for more pages to add to the queue; and
2. an idea of Keith's: application threads only get involved with eviction when their session is quiet if they have missed in the cache recently while it was full.

For me, this branch helps quite a lot with the workload from #1068 (separately or together with #1082).
Clearing the flag here may actually be the real win.  What this does is make sure we don't get aggressive when there are multiple threads helping with eviction.

This may not be quite right (I suspect some pathological test/format configurations may get stuck), but getting aggressive when we don't need to is a problem, because we try lots of eviction that is likely to either fail or be inefficient.