Check that timeout times are derived from times in messages, not the system time.

[jmalloc]

This one might be near impossible, but it should at least catch some common cases like using time.Now().

I'm not 100% confident this is something we want to enforce all the time, but it might be better to produce a diagnostic about it so that its brought to people's attention, then relax it if we find valid use cases for using the system time.