Use steady_clock to measure connection timeout

[jureviciusr]

The connection timeout in the connection class was using the std::chrono::system_clock which is not monotonic and susceptible to time jumps when system wall clock is adjusted, see here: https://en.cppreference.com/w/cpp/chrono/system_clock

This can lead to connection timeout incorrectly evaluated. std::chrono::steady_clock should be used instead, as it is not susceptible for wall clock adjustments and should be preferred for time duration measurements: https://en.cppreference.com/w/cpp/chrono/steady_clock

The PR changes are the following:

• Remove the millis() public method from utils
• Replace integer millisecond with std::chrono::milliseconds for a more explicit type
• Use steady_clock to sample timestamps to measure connection timeout

[DanMesh]

I would hope it's safe to remove this public helper function.

If we're worried, we could keep and at least make it return a steady clock time.

[jureviciusr]

I don't know how far reaching is this, within this project it was only used only once. We can deprecate the method first, so it would print a compile-time warning with a clear message and get deleted with next version bump, e.g.

    [[deprecated("millis() method uses non-monotonic clock source and it will be removed in the next release.")]] inline uint64_t millis() noexcept {
        return std::chrono::duration_cast<std::chrono::milliseconds>(
                std::chrono::system_clock::now().time_since_epoch()).count();
    }

I would strongly suggest to remove it, since it's up to the developer to select an appropriate clock source for the task.