Make sure LRU eviction drains before checkpoints start.  Don't flip the "no eviction" flag if it is already disabled.
Don't lock internal pages during checkpoints: we now guarantee all eviction threads will see the "checkpointing" flag.
Don't deepen trees when discarding files from cache.
Merge pull request #925 from wiredtiger/new-split-mjc-fixes

Some minor fixes for checkpoints and eviction with new splits