Changes:
- separate ArenaPoll and ArenaStep code paths;
- simplify ArenaPoll;
- loop calling TracePoll to catch-up;
- update pollThreshold correctly, depending on whether we have no work and are sleeping, or have work and are advancing the 'clock' by one unit. If there's no work, don't keep checking. Avoid multiple calls to ClockNow().
- ArenaStep should NOT change pollThreshold -- that's ArenaPoll's business. This means ArenaStep may advance, but not retard, trace work.
zcoll: 100MB is a more sensible arena size than 0.5 MB
test-runner.py: run zcoll; on w3i3, use m9 (=VC9.0) compiler
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Change: 169904
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