Correctly keep track of where we are in the quicserver request buffer #811

Job	Run time
basic_clang	10m 2s
basic_gcc	7m 38s
check-ansi	4m 16s
address_ub_sanitizer	14m 11s
check_docs	1m 8s
check_update	1m 52s
enable_brotli_dynamic	7m 23s
enable_non-default_options	7m 38s
enable_zstd_dynamic	7m 9s
fips_and_ktls	7m 46s
memory_sanitizer	18m 58s
minimal	4m 15s
no-deprecated	7m 18s
non-caching	17m 22s
no-shared (ubuntu-latest)	5m 22s
no-shared (macos-latest)	8m 26s
threads_sanitizer	5m 50s
buildtest	11m 37s
enable_brotli_and_asan_ubsan	13m 58s
enable_brotli_and_zstd_dynamic	7m 2s
enable_zstd_and_asan_ubsan	13m 36s
enable-tfo (ubuntu-latest)	6m 57s
enable-tfo (macos-latest)	10m 44s
external-test-cf-quiche	7m 9s
external-test-pyca (1.51.0, 3.9)	10m 51s
external-tests	17m 22s
legacy	5m 45s
no-legacy	7m 48s
out-of-source-and-install (ubuntu-latest)	10m 49s
out-of-source-and-install (macos-latest)	15m 4s
	4h 35m 16s

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