[avalanche] Use SteadyMilliseconds where possible

Summary:
std::chrono::time_point<std::chrono::steady_clock> is a mouthful and doesn't hint about the expected resolution. Use SteadyMilliseconds where due instead as this is already available in our time.h header.
The Now() function is made constexpr to make sure there is no performance cost over using the standard now() function.

There is no change in behavior.

Depends on D14553.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14554