Improve HTTPHeader normalisation performance for large inputs

[glbrntt]

Motivation:

Header normalisation is expensive when the input headers contain a large
number of connection header values.

Modifications:

• When the headers to normalise contain more than 32 connection header
  values use a set to provide constant time lookup, rather than linear.

Result:

Better perf for HTTPHeader normalisation with large connection header
value lists.

[glbrntt]

@swift-nio-bot test perf please

[swift-server-bot]

performance report

build id: 21

timestamp: Tue May 17 09:43:08 UTC 2022

results

name	min	max	mean	std
1_conn_10k_reqs	1.6607459783554077	1.700727105140686	1.6850006937980653	0.01528078307876625
encode_100k_header_blocks_indexable	0.14468300342559814	0.14531099796295166	0.1451084017753601	0.00019574398671356246
encode_100k_header_blocks_nonindexable	0.33011603355407715	0.3314650058746338	0.3309402108192444	0.0004103333324755851
encode_100k_header_blocks_neverIndexed	0.3330259323120117	0.3353539705276489	0.33367558717727663	0.0006497830993229124
decode_100k_header_blocks_indexable	0.10497593879699707	0.10560905933380127	0.1053286075592041	0.000228500360129959
decode_100k_header_blocks_nonindexable	0.12923407554626465	0.1303880214691162	0.12971220016479493	0.00034571157010218476
decode_100k_header_blocks_neverIndexed	0.1289670467376709	0.12961196899414062	0.1293125033378601	0.0002540544436384093
hpackheaders_canonical_form	0.030851006507873535	0.03143799304962158	0.031002700328826904	0.00021552581119081013
hpackheaders_canonical_form_trimming_whitespace	0.040794968605041504	0.04140293598175049	0.04097169637680054	0.00021835044364994555
hpackheaders_canonical_form_trimming_whitespace_short_strings	0.03802001476287842	0.03951394557952881	0.03843930959701538	0.00043811663799079366
hpackheaders_canonical_form_trimming_whitespace_long_strings	0.04514908790588379	0.04574394226074219	0.04535740613937378	0.0002535848025536832
hpackheaders_normalize_httpheaders_removing_10k_conn_headers	0.05470395088195801	0.0552980899810791	0.0549164891242981	0.00021631257253239196
hpackheaders_normalize_httpheaders_keeping_10k_conn_headers	0.03927600383758545	0.03979194164276123	0.03942561149597168	0.00017836633929544692
huffman_encode_basic	0.7919390201568604	0.8101199865341187	0.7946073889732361	0.005608274018308786
huffman_encode_complex	0.5319410562515259	0.5357859134674072	0.5328487038612366	0.0012002314451067637
huffman_decode_basic	0.057235002517700195	0.05780899524688721	0.057450401782989505	0.00020809817280214294
huffman_decode_complex	0.04532897472381592	0.04599606990814209	0.04555720090866089	0.00022831002950002882
server_only_10k_requests_1_concurrent	0.3517889976501465	0.35248804092407227	0.3521202802658081	0.0002654268948074409
server_only_10k_requests_100_concurrent	0.28675997257232666	0.2875009775161743	0.2872292876243591	0.0002297801752170679
stream_teardown_10k_requests_100_concurrent	0.1527460813522339	0.15355408191680908	0.15320420265197754	0.00031405841619329346

comparison

name	current	previous	winner	diff
1_conn_10k_reqs	1.6850006937980653	1.6798397064208985	previous	0%
encode_100k_header_blocks_indexable	0.1451084017753601	0.14162989854812622	previous	2%
encode_100k_header_blocks_nonindexable	0.3309402108192444	0.3299762964248657	previous	0%
encode_100k_header_blocks_neverIndexed	0.33367558717727663	0.3293057084083557	previous	1%
decode_100k_header_blocks_indexable	0.1053286075592041	0.10489749908447266	previous	0%
decode_100k_header_blocks_nonindexable	0.12971220016479493	0.13030190467834474	current	0%
decode_100k_header_blocks_neverIndexed	0.1293125033378601	0.13055150508880614	current	0%
hpackheaders_canonical_form	0.031002700328826904	0.03136119842529297	current	-1%
hpackheaders_canonical_form_trimming_whitespace	0.04097169637680054	0.04263371229171753	current	-3%
hpackheaders_canonical_form_trimming_whitespace_short_strings	0.03843930959701538	0.03957270383834839	current	-3%
hpackheaders_canonical_form_trimming_whitespace_long_strings	0.04535740613937378	0.04882251024246216	current	-7%
hpackheaders_normalize_httpheaders_removing_10k_conn_headers	0.0549164891242981	7.75164178609848	current	-99%
hpackheaders_normalize_httpheaders_keeping_10k_conn_headers	0.03942561149597168	14.962627184391021	current	-99%
huffman_encode_basic	0.7946073889732361	0.7547392010688782	previous	5%
huffman_encode_complex	0.5328487038612366	0.500220799446106	previous	6%
huffman_decode_basic	0.057450401782989505	0.05857760906219482	current	-1%
huffman_decode_complex	0.04555720090866089	0.04356350898742676	previous	4%
server_only_10k_requests_1_concurrent	0.3521202802658081	0.3536600828170776	current	0%
server_only_10k_requests_100_concurrent	0.2872292876243591	0.28573299646377565	previous	0%
stream_teardown_10k_requests_100_concurrent	0.15320420265197754	0.15034650564193724	previous	1%

significant differences found

[Lukasa]

Really nice change!