Add statistics to track eviction server candidate list sizes.

Also back out a few bits of eviction-under-load that weren't helping.

This branch doesn't improve my overall throughput on the #1`068 workload.
Update how multiple eviction threads interact.

* Don't do eviction in main eviction thread, if multiple threads configured.
* Don't do eviction in main eviction thread, if application threads are
  waiting for candidates.
* Only have additional eviction threads do a condition wait if there isn't
  any work to do.
Merge branch 'eviction-under-load' into eviction-stats
Fix build warning.
Merge pull request #1089 from wiredtiger/eviction-stats

Add statistics to track eviction server candidate list sizes.