Task manager wip rebased

I rebased the work Arturo and I did on ooni-probe's new task manager on top of the current master. There's a lot of new stuff here, and someone other than Arturo or I should take a good look at it.

I pulled the fixes I did to some of the nettests (captive portal, parasitic traceroute) out of this branch (originally they were in my taskManagerWIP branch) -- those will go into separate pull requests.