Revert part of the previous change: don't drop the handle list lock when scanning for pages.  We're only doing a trylock, so if we find some candidate pages, then fail to get the lock when we retry, we have wasted work.

refs #1391
Avoid stalls with LSM trees when the cache is full.

1. Don't block indefinitely on a full cache if we're holding the handle list lock.

2. If the cache is full and we're trying to discard chunks from an LSM tree, have the eviction server give up the data handle list lock so the discard doesn't stall.

refs #1391
Have the eviction server back off if it isn't making progress to give LSM a better chance to discard trees from cache.