Request fewer GitHub releases and cache string URLs in Netkan

[HebaruSan]

Problem

New inflation error for HalfRSS-Textures2048: GitHub API rate limit exceeded.
New inflation error for HalfRSS-Textures4096: GitHub API rate limit exceeded.
New inflation error for HalfRSS-Textures8192: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-Afterburner: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-AutoBalancingLandingLeg: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-DisableTempGauges: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-DailyFunds: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-DestroyAll: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-EditorCamUtilities: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-FillSpotsWithTourists: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-FPSLimiter: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-FPSViewer: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-KerbalScienceExchange: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-ModifiedExplosionPotential: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-PhysicalTimeRatioViewer: GitHub API rate limit exceeded.
New inflation error for KerboKatzSmallUtilities-RecoverAll: GitHub API rate limit exceeded.
New inflation error for KerboKatzUtilities: GitHub API rate limit exceeded.

Cause

The exact reason this happens is uncertain, but we suspect we're triggering GitHub's abuse prevention limits:

• We're probably requesting too many GitHub releases at once (100)
• We're probably requesting the same API URLs too many times in a short period

Changes

• Now we request 10 GitHub releases instead of 100, and if we need more (rare!), we request 10 more, and so on. This will cut down on the server load to handle our GitHub API requests.
• Now CachingHttpService.DownloadText caches its results for 3 minutes, so any repeated requests for the same URL (e.g., if we inflate all of the KerboKatz modules in rapid succession) will only hit the server once. This will avoid the possibly abuse-y behavior of hitting the same URL over and over, and also probably speed up the Inflator a little bit (since memory is a lot faster than network).

Fixes #2949 (hopefully!).

[HebaruSan]

I want to see whether this helps, but my reviewers are busy for holiday break. 🎄
Merging and will keep an eye and revert if the bot stops working.

[DasSkelett]

Hehe. I did take a short look over the code just now, but without testing or anything.
It looks good, maybe it could've used some debug log statements to see whether the cache has been hit or not, but function-wise I think it's good.

[HebaruSan]

Nailed it!

So just like in #2928, if we get rate limited when the limit is not exceeded, it's because of repeated calls to the same URL. Which should be impossible now.

[HebaruSan]

Hmm, I think this froze the Inflator after about 2 days, maybe we used too much memory. We can purge more pro-actively, but that has to be done carefully to avoid taking up a ton of CPU.