do get requests less often and more intelligently

[thomasmitchell]

Certs that aren't expiring for a long time are less likely to be renewed at any given moment. So after we perform a list, if we know about some of the returned paths already and they correspond to certs that aren't expiring in awhile, then we should poll them on a slower schedule. If we're a good polling citizen, then we can justify refreshing more often for the important cases.

the refresh endpoint would need an argument to mean refreshing everything, regardless of impending doom.