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.
Fix wtperf errors detected by valgrind: a buffer size calculation didn't include the NUL terminator, and some memory wasn't free.  While there, simplify cleanup by relying on the semantic that free(NULL) is a noop.

refs #633
Change the wtperf "medium" LSM config to use larger chunks and create Bloom filters for unmerged chunks.
This means less work for the merge thread, and insurance in case merges still fall behind.  If this helps improves performance, we should consider changing the LSM default configuration in future to match.