Update comment.
__wt_conn_btree_apply should not process at checkpoint handles.

Refs #709.

There are currently two callers of __wt_conn_btree_apply:
 * Checkpoint, which skips handles open on checkpoints.
 * Global statistics dump, which ignores the checkpoint flag on handles it
   retrieves, so will end up dumping statistics for a file twice if a
   checkpoint handle is open.
Only clear the "no eviction" flag in LSM after the checkpoint has completed.  Otherwise, checkpoint will try to forcible evict the leaf page.

refs #713