Avoid double loading gauge data on server

[ecc521]

Use Canadian gauges Last-Modified to not load the same files twice.

Also use USGS's modified since API feature (however it works)

Not sure either of these is high priority. Not much USGS data is requested anymore (3 hours for most gauges), and everything is compressed to tiny sizes.

Canada doesn't compress anyways, though I've sent in multiple reports about it. They don't seem to care, and Google Cloud doesn't care about ingress. (If Canada doesn't care about a 20x decrease that they can do easily, a harder 4x decrease on my end seems silly)