return max-age for etag responses

[rafaelcr]

If these etags are all time based, we could possibly implement very efficient caching with cache-control: max-age=x -- the browser could reliably use its cached data (or cloudflare's) without having to revalidate against the API. You'd want to only do this when the datetime is old enough to avoid bitcoin reorgs.

[chresko]

I like this approach. We should bump up the priority if we can also demonstrate the value of max-age cache control in our Explorer. @janniks thoughts?

[smcclellan]

The cache is already working well, deprioritizing for now.