net, refactor: Rename CNetMessage::m_command with CNetMessage::m_type

Summary:
A message is not a command, but simply a message of some type. Continuation of D8460

scripted-diff: Rename CNetMessage::m_command with CNetMessage::m_type

-BEGIN VERIFY SCRIPT-
sed -i 's/std::string m_command;/std::string m_type;/g' ./src/net.h
sed -i 's/* command and size./* type and size./g' ./src/net.h
sed -i 's/msg.m_command/msg.m_type/g' ./src/net.cpp ./src/net_processing.cpp ./src/test/fuzz/p2p_transport_serialization.cpp
-END VERIFY SCRIPT-

net, refactor: Drop tautological local variables

This is a backport of core#24078

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14772