imported patch multiple-eviction-threads
Only try to allocate memory for eviction workers if they are configured.
Merge branch 'develop' into multiple-eviction-threads
Merge branch 'develop' into multiple-eviction-threads
Merge branch 'develop' into multiple-eviction-threads

Conflicts:
	src/btree/bt_evict.c
Merge branch 'develop' into multiple-eviction-threads
Merge branch 'develop' into multiple-eviction-threads
Merge pull request #918 from wiredtiger/multiple-eviction-threads

Support multiple eviction threads