Have the eviction server concentrate on walking when there are other threads waiting to do eviction.  This is an attempt to avoid cases where the eviction server is busy doing eviction and stalls all other threads.
Only threads that try to read pages when the cache is full should block for eviction.
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 'develop' into eviction-under-load

Manually merged:
	src/include/session.h
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.
Merge branch 'develop' into eviction-under-load
Merge pull request #1087 from wiredtiger/eviction-under-load

Improve eviction under load.