refactor: s/command/msg_type/ in CNetMsgMaker and CSerializedNetMsg

Summary:

Follow-up PR for #18533 -- another small step towards getting rid of the
confusing "command" terminology. Also see PR #18610 which tackled the
functional tests.

Backport of core PR18937.

Depends on D8460.

The change to process_messages.cpp is not ported as this PR is
deliberately ported out of order. The process_messages fuzzer was added
in D8376 and reverted because it caused a TSAN failure; this diff is a
dependency for the fix.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8462