Modify and improve config change callback system

[nickelization]

These changes break backwards compatibility with the old config callback API, but the new system is much easier to use in that it automatically deals with use of the --node and --all flags and calls the callbacks in the correct places. It also allows the callbacks to add output messages that can be displayed back to the user. In the case of the --all flag, all the results will now be displayed back to the user in a table.

This PR also updates the documentation appropriately, and adds extensive unit tests for the clique "set" command.

[ian-mi]

Good work, I will wait for a rebase to squash some of these commits before going ahead with the merge.

[nickelization]

@ian-mi Alright, after much git wrangling, I think that's about as clean and tidy as I'm going to get it without having to start manually resolving merge conflicts. GitHub seems to have gotten a bit confused by all my rebases and force pushes, because it's showing the commits out of order now, so be careful to get the correct commit hash when you +1 it for Bors. (Currently 14e01b0918a94cf741265ab23affa2132869a282).

[ian-mi]

looks good, +1 14e01b0

[nickelization]

I think Bors needs the +1 to be the first thing in the comment and on its own line. Let me try one more time...

[nickelization]

:+1: 14e01b0

[nickelization]

@borshop merge