Changes to tune eviction loops based on profiling.  Track more pages, but only sort the maximum number of entries we have actually seen.
Merge doesn't need to clear pages from the eviction queue or clear the eviction position: we're using the same locking scheme as normal eviction.
Merge branch 'develop' into eviction-tuning

Conflicts:
	src/btree/bt_evict.c
Merge branch 'develop' into eviction-tuning

Conflicts:
	src/btree/bt_evict.c
We have to remove live pages from the LRU queue during merge: we check before evicting that page == page->ref->page, which merge violates briefly.
Merge branch 'develop' into merge-no-clear
Merge pull request #508 from wiredtiger/eviction-tuning

Changes to tune eviction loops based on profiling.
Re-check the "no eviction" flag while holding the eviction walk lock.  This flag is used to enforce exclusive access when a handle is being closed, and a race meant that eviction could reference freed memory.
closes #517
Merge pull request #529 from wiredtiger/merge-no-clear

Merge doesn't need to clear pages from the eviction queue or clear the eviction position
Keep track of the maximum slot used in the LRU queue in a volatile, monotonic variable, so we can never miss a page when scanning.