If we're discrading a page with a hazard pointer, assert failure in
DIAGNOSTIC mode.
Change __wt_conn_btree_apply_single to match the requested handle using
both the URI and the checkpoint.  (Because we're only matching bulk-load
files, the checkpoint we're requested to match on should always be NULL;
rather than enforce that, change __wt_conn_btree_apply_single() to match
only bulk-loaded files -- __wt_conn_btree_apply_single() has other
problems this commit doesn't solve, but this cleans up a bug and lets
us move forward.

This change originally committed, and then backed out in #674 (ef3f602).

Ref #679.
Delaying the set of session.dhandle introduced a NULL dereference
bug, fix it.
Merge pull request #681 from wiredtiger/bulk-handles

Match on both the URI and the checkpoint name in __wt_conn_btree_apply_single.