Use typed channels

[webbnh]

Changes introduced with this PR

This is a follow-on to #96: if the channel instances had been properly typed (to wit, as receive-only), it might not have been possible to close them in the first place (since an attempt to close a receive-only channel results in an error), and that might have alleviated some trouble. So, this PR modifies the Client.Execute() interface (and its implementation and subroutines) to declare the receivedSignals parameter as a receive-only channel, so that its usage is clearer.

In the process of producing that change, it became apparent that we no longer need the handleStepComplete() function and the runningSignalReceiveLoops map, so these were removed, as well as the code supporting them.

This PR also includes three other sets of changes:

• The first picks lint (and fixes some real issues):
  • There were three places where we were using the %w formatting verb outside of the context of a fmt.Errorf() call:
    • one I fixed by changing it to %v; and,
    • two I fixed by factoring out the formatting, since it was being shared with logging.
  • There was a place where we were missing a formatting verb (it was typed as '&s'; I replaced it with %q).
  • And, there were a couple of other, minor lint complaints that I addressed.
• The second removes an existence check which is unneed post Fix Channel Close Behavior #96.

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[jaredoconnell]

Looks good. It probably makes sense to address the comments that Dave pointed out.

[jaredoconnell]

Looks good. I tested it with the engine and there are no new problems.

[dbutenhof]

More simplerest ...