fa3365430c net: Use mockable time for ping/pong, add tests (MarcoFalke)
faab4aaf2f util: Add count_microseconds helper (MarcoFalke)
Pull request description:
Switch `CNode::m_ping_start` and `CNetMessage::m_time` to mockable time, so that tests can be added.
Mockable time is also type-safe, since it uses `std::chrono`
ACKs for top commit:
jonatack:
Code review re-ACK fa33654 re-read code, verified rebase per `git range-diff 4b5c919 fa94d6f fa33654`, previous tested ACKs still valid
troygiorshev:
ACK fa3365430c
Tree-SHA512: 7d632bd6019ce7c882029e71b667a61517e783af82755a85dd979ef09380934e172dec8b8f91d57b200a30a6e096aeaf01f19fee7f3aed0e0e871c72eb44d70e