Periodic log using time instead of count

[darbitman]

The macro LOG_EVERY_N and its siblings use a counter to display a message every Nth times it gets called. This is great for limiting logs, but in systems where the log calls are not only infrequent but also varies it would be beneficial to log every time period T instead. When the log calls are very frequent this will rate limit to 1 log entry per time T, and when they become infrequent, it will turn into at most 1 log entry per time T.

I plan on adding this feature.