use SafePull at top of all flowgraph goroutines

[mccanne]

After the nuke-driver branch is merged, we need to make sure all flowgraph goroutines use SafePull as the new expr package calls panic for unrecoverable errors and we need to catch these.

[philrz]

@mccanne and @nwt think this is done, but it could use an audit before we close it out.

During an issue scrub, we also came across #3387 which sounds like effectively the same topic, so that's now been closed as a duplicate and should can be checked as part of the same audit. The text from that issue:

We need to make sure every goroutine in a flowgraph is protected by a recover() which can be implemented using proc.Catcher. We should also refactor all the goroutines saw they are all launched and managed in a consistent way to the extent that this is possible.