support eviction_dirty_trigger for WiredTiger

Discussion started in SERVER-16665. WT supports eviction_dirty_target, eviction_target and eviction_trigger. I also want a setting for eviction_dirty_trigger. With a 144G RAM server I don't want 140G of dirty pages in the WT cache but I need eviction_dirty_trigger and eviction_dirty_target to enforce that. Otherwise, the percentage of dirty data in the cache will approach 100% and if eviction_dirty_trigger is small then eviction might run for a very long time.