MINIFICPP-1116 - Fix error reporting in C2Agent

[szaszm]

• Missing CoapProtocol will now result in a fatal error (exception)
  instead of segfault. Error reporting is improved.
• C2Agent::protocol_ is now properly deleted
• remove dead code
• avoid unnecessary string copies in loop

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[szaszm]

The AppVeyor failure does not seem to be related to the changes in this PR. Is it unrelated or am I missing something?

[bakaid]

Generally looks good, nice catches, a few issues in inline comments.

[szaszm]

update:

• avoid override
• use minifi::Exception
• reorder ctor initializer list to match initialization order

[arpadboda]

Thanks, LGTM!

[bakaid]

LGTM, let's wait for the CI to finish.