Modernize quic_types.h, no functional change As per the style guide <http://go/cstyle#Aliases> new code should use "using" instead of "typedef". This CL cleans up our usage in quic_types. It also adds comments and removes some unused types. gfe-relnote: cleanup, no behavior change, not flag protected PiperOrigin-RevId: 308715980 Change-Id: Ib5b61f6299ca08b29011cbf380956f8acf80cc91