Make all `const` duration values more explicit.

[jstuczyn]

Currently all default config duration values are defined in u64 and are treated as if they were milliseconds. This could be made more explicit now that Duration can be constructed in compile time. So instead of
const DEFAULT_MESSAGE_STREAM_AVERAGE_DELAY: u64 = 500;
we could have:
const DEFAULT_MESSAGE_STREAM_AVERAGE_DELAY: Duration = Duration::from_millis(500);