Add a new per-session flag, WT_SESSION_CAN_WAIT

@michaelcahill, here's the change we discussed, plus a couple of semi-related odds-and-ends (all on separate commits, so you can cherry-pick or revert easily if you want).

Reference #1152.
To be sure I understand, would: "The checkpoint thread benefits from being able to run intermediate sync operations - allow it to wait so they aren't skipped." be an alternative description here?