Backport AltSvc/SOCKS patch to ESR60

Patrick McManus wrote a patch for using Alternate Service with SOCKS that landed in Firefox 62 (https://bugzilla.mozilla.org/show_bug.cgi?id=1463509). This would be useful for us, especially for AltSvc onions. So we'd like to backport to ESR60. Turning this on will require completing our audit of HTTP2 and AltSvc mechanism in particular to make sure it respects FPI.

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