[Basic] Increase background task max.
[Basic] Increase background task max.

 - Various testing seems to indicate the relative cost of burning backgrounds
   threads isn't that high, it seems safe to go somewhat higher. We do still put
   a max cap on things, though.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild