Refactor handshake msg header parsing etc. #318

Job	Run time
check_docs	1m 5s
check_update	1m 13s
basic_clang	1m 19s
basic_gcc	1m 10s
check-ansi	1m 6s
minimal	48s
no-deprecated	1m 13s
self-hosted	0s
address_ub_sanitizer	1m 12s
fuzz_tests	1m 11s
memory_sanitizer	1m 52s
non-caching	1m 9s
no-shared-macos (macos-13)	7m 23s
no-shared-ubuntu	1m 14s
no-shared-macos (macos-14)	4m 56s
enable_non-default_options	1m 4s
external-test-cf-quiche	1m 34s
external-tests	1m 37s
external-test-pyca (1.51.0, 3.9)	1m 19s
full_featured	1m 33s
legacy	53s
no-legacy	1m 8s
out-of-readonly-source-and-install-macos (macos-13)	12m 54s
out-of-readonly-source-and-install-ubuntu	1m 17s
out-of-readonly-source-and-install-macos (macos-14)	7m 12s
threads_sanitizer	1m 23s
	58m 45s

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