Be more aggressive with LSM throttling if checkpoints fall behind.
Don't pin chunks in cache when cursors are open on LSM trees.

We only need to hold the position while an update operation is in
progress.
Bump the primary chunk reference count before releasing the LSM lock.
Merge pull request #548 from wiredtiger/lsm-many-threads

Be more aggressive with LSM throttling if checkpoints fall behind. Don't pin chunks in cache just because cursors are open on LSM trees.