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.
I believe moving this means that the eviction server can start additional worker threads before their sessions are created. Guess I should have added a comment..