Enhance forced eviction to allow multiple pages to be queued.
Merge pull request #496 from wiredtiger/multi-forced-eviction

Enhance forced eviction to allow multiple pages to be queued.
Cleanup unused variables.
bt_evict.c: In function '__evict_forced_pages':
bt_evict.c:455:11: error: variable 'force_page' set but not used [-Werror=unused-but-set-variable]
bt_evict.c:451:12: error: variable 'force_btree' set but not used [-Werror=unused-but-set-variable]
Get the eviction lock before finding a slot for forced evction.