Update the LSM throttle calculations.

* We were using the wrong check to see if any chunks had been checkpointed
  in the current tree.
* A couple of other updates for readability.
Remove unused loop variable.
Merge pull request #1044 from wiredtiger/lsm-throttle-tune

Improve LSM throttling.
Fix a bug just introduced to LSM. Only look at on disk chunk if it exists.
Exercise all isolation levels in test/format.

motivated by reviewing #1040