Don't attempt to split pages when checkpoints are active in a tree.

refs #1419, SERVER-16336
Only do in-memory splits on large, dirty pages.
Don't try in-memory splits when closing a tree.
Merge pull request #1427 from wiredtiger/split-fixes

Fixes for in-memory splits
Reduce the chunk size in evict-lsm wtperf workload.

We aren't trying to stress out what happens when the cache gets full.