Reject inappropriate ipt_mgr configuration changes; Accept and implement the ones that we can.

For example, changing the state_dir would result in terrible lossage. Some checks need to be added to the reconfigure logic.

MUST because unbounded lossage might result.

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Expanding this ticket to note that there are also places where we could implement certain changes at runtime, but we do not. I will note those with TODO #1209 as well. We can be flexible about what we implement now and what we forbid, but we should make sure that every change is either supported or forbidden.

—@nickm