Invalidate non-background url sessions

[morganchen12]

• Fixes issue where sessions without a background task ID wouldn't be invalidated (Memory leak in GULNetworkURLSession #1917, Memory leak in GULNetwork.m #2060)
• Invalidates sessions immediately upon task completion
• Adds URL cache to some non-background tasks

I'm worried this solution is insufficient for instances of GULNetworkURLSession that kick off multiple requests simultaneously since the _URLSession ivar will be overridden.

Changes mirrored at cr/220718996

[morganchen12]

style.sh fails with an unclear error message.

morganchen-macbookpro:firebase-ios-sdk morganchen$ scripts/style.sh 
No such file or directory
No such file or directory

[paulb777]

I've seen that message when there's extra files in the repo, but it still successfully runs. Check git status to see if any files got updated.

[paulb777]

The change looks good to me assuming it clears travis and internal tests. Let's also get a review from @ryanwilson

[ryanwilson]

Seems reasonable to me, please make sure this is tested via the repro steps from #2060

[paulb777]

Please update https://github.com/firebase/firebase-ios-sdk/blob/master/GoogleUtilities/CHANGELOG.md

[morganchen12]

Tested manually, leaks went away.