Hg polling can hang indefinitely

If the hg process for e.g. 'hg incoming' hangs for whatever reason (network
issues?), Hudson will just cease to build that job. The only indication is the
"There are more SCM polling activities scheduled than handled" warning.

Probably there should be some timeout on polling operations, such as 1 hour.
Builds can already be set to timeout so these probably need no special handling.