HTTP/1 headers simplification & cleanup

[weissi]

Motivation:

The HTTP/1 headers were quite complicated, CoW-boxed and exposed their
guts (being a ByteBuffer). In Swift 5 this is no longer necessary
because of native UTF-8 Strings.

Modifications:

• make headers simply [(String, String)]
• remove HTTPHeaderIndex, HTTPListHeaderIterator, etc

Result:

• simpler and more performant code
• nice speedup:

before:

$ wrk http://localhost:8888
Running 10s test @ http://localhost:8888
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   162.88us   44.81us   1.79ms   75.12%
    Req/Sec    28.75k     2.22k   32.83k    80.20%
  577919 requests in 10.10s, 28.66MB read
Requests/sec:  57218.56
Transfer/sec:      2.84MB
$ tcpkali -em "GET / HTTP/1.1\r\nHost: google.com\r\n\r\n" -r 10000000000000 --latency-marker "HTTP/1.1" 127.0.0.1:8888
Destination: [127.0.0.1]:8888
Interface lo0 address [127.0.0.1]:0
Using interface lo0 to connect to [127.0.0.1]:8888
Ramped up to 1 connections.
Total data sent:     15.2 MiB (15916740 bytes)
Total data received: 20.8 MiB (21827312 bytes)
Bandwidth per channel: 30.174⇅ Mbps (3771.7 kBps)
Aggregate bandwidth: 17.449↓, 12.724↑ Mbps
Packet rate estimate: 40324.3↓, 1101.8↑ (1↓, 37↑ TCP MSS/op)
Message latency at percentiles: 672.7/691.5/695.5 ms (95/99/99.5%)
Test duration: 10.0071 s.

after:

$ wrk http://localhost:8888
Running 10s test @ http://localhost:8888
  2 threads and 10 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   151.39us   47.03us   1.40ms   80.82%
    Req/Sec    30.72k     3.80k   53.58k    67.16%
  614191 requests in 10.10s, 30.46MB read
Requests/sec:  60814.41
Transfer/sec:      3.02MB
$ tcpkali -em "GET / HTTP/1.1\r\nHost: google.com\r\n\r\n" -r 10000000000000 --latency-marker "HTTP/1.1" 127.0.0.1:8888
Destination: [127.0.0.1]:8888
Interface lo0 address [127.0.0.1]:0
Using interface lo0 to connect to [127.0.0.1]:8888
Ramped up to 1 connections.
Total data sent:     18.1 MiB (18945684 bytes)
Total data received: 24.9 MiB (26158600 bytes)
Bandwidth per channel: 36.077⇅ Mbps (4509.6 kBps)
Aggregate bandwidth: 20.923↓, 15.154↑ Mbps
Packet rate estimate: 47387.0↓, 1316.9↑ (1↓, 37↑ TCP MSS/op)
Message latency at percentiles: 557.5/571.5/576.3 ms (95/99/99.5%)
Test duration: 10.0019 s.