Bulk checkpoint changes

@michaelcahill, (so far) these are changes I made while staring at the bulk-load & checkpoint code for #1114. There's nothing tricky here yet, it's just cleanup.  If you want to review/merge, that'd be cool with me, I can open a new branch.
This change is problematic for LSM: it makes the current in-memory chunk evictable, which we don't want.  We could turn off `bulk_load_ok` inside `__wt_btree_evictable` instead to avoid this.