Clean up/fix locking in handleReturn

[zenhack]

This cleans up/fixes some of the locking towards the end of handleReturn; previously this was super convoluted, in part because we'd just put the whole rest of the function in a giant go func() {...}() at some point to solve a deadlock without cleaning it up in a more structural way; see the now-deleted TODO(cleanup).

This also adds a few comments related to the reasoning behind some conditionals, which are a bit obtuse. I think this probably could be cleaned up further, but this maintains the existing behavior while documenting it a bit better at least.

Finally, while doing the above I noticed that sendMessage() was being called without holding c.lk, so I added a call to syncutil.With to fix that.

After this patch there are only three remaining calls to syncutil.Without in the rpc package.

[zenhack]

You might want to look at the individual commits instead of the whole diff, which is horrible. The latter commit indented a big block of code which git never seems to understand very well.

[zenhack]

Source of the failure isn't immediately obvious to me; I'll stare at it in the morning (though I would hazard a guess it's been there and is not related to this patch).

[zenhack]

Last failure looks like #377, so not the same bug. Once we've nailed down these more structural improvements, we should probably do another pass of "get CI to stop failing"