Start and stop eviction worker threads based on demand.

Refs #1116
Fix an off by one in dynamic eviction threads.
Merge branch 'develop' into dynamic-evict-workers

Conflicts:
	src/btree/bt_evict.c
	src/conn/conn_cache.c
	src/include/cache.h
Don't have eviction workers close sessions.
Merge branch 'develop' into dynamic-evict-workers
Lint
Make dynamic eviction workers the default. #1143
Fix a bug in new eviction thread parsing.

Update example file and documentation as well.
Add upgrading doc for eviction thread API change.
Fix an issue cleaning up eviction after a failed open.
whitespace
Move the new eviction settings into a group.  We can transition older settings into the group in the next major release: that will break existing applications.
Change ex_all.c to match grouping of eviction settings.
Engage brain: applications have to use <group>=(<key>) syntax for nested configs.
Merge pull request #1143 from wiredtiger/dynamic-evict-workers

Start and stop eviction worker threads based on demand.