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.
I'm hesitant to touch `bulk_load_ok` inside of `__wt_btree_evictable`, but I'm also unhappy that trying to simplify eviction made LSM more complicated.   Overall, I guess I still prefer this version, I'll go ahead and merge.