Adding Lancache support to the CDN Client.

[tpill90]

Added automatic Lancache detection to the CDN client. If a Lancache is on the local network and the trigger domain is poisoned, then SteamKit will direct all traffic through the cache instead of Valve's CDNs.

This behavior should match what the current behavior in Steam is.

[Dimensional]

This has been sitting here for a while, but I realized something that needs to be taken into consideration is that if the LanCache has a bad file saved, like a file got corrupted in storage or network issues resulted in a 0-length file, this setup would constantly request the file over and over again, constantly getting the same bad file and delaying downloads.

This is a flaw even with Steam when it grabs from LanCache, and requires reviewing the access logs, identify the files being requested repeatedly, and then have to either locate the cached files and delete them, or do what steam-lancache-prefill does and append ?nocache=1 to the end of the url string in order to force LanCache to discard what it has saved and grab from upstream again.

[xPaw]

I thought the method to detect lancache automatically was fine, by doing a look up. Just make it assign result into UseLancacheServer if available (and leave calling the method to consumers).

[tpill90]

I thought the method to detect lancache automatically was fine, by doing a look up. Just make it assign result into UseLancacheServer if available (and leave calling the method to consumers).

Put the lancache detection into a method for users to call, should match what you were expecting now.

[tpill90]

@xPaw Should I squash this branch prior to merging now that it's approved?

[xPaw]

We can squash on merge. Did you test this pr with Depot downloader?

[tpill90]

Yep. Tested again just now with the -use-lancache flag I was adding to DD. Top pane is me running DD w\ the flag against this PR branch. Bottom pane is the access.log from Lancache showing that it is pulling from the cache.

[tpill90]

Fixed the latest comment, rebased, and pushed again.

[tpill90]

@yaakov-h You mind giving this a review when you have a moment? We should be good to merge at this point.