Improve performance, reuse server params for same connection

[clue]

This PR improves the HTTP server performance by reusing the socket addressses stored in the server params for the same connection. This helps avoiding unneeded getpeername() and getsockname() syscalls that show a noticeable performance impact especially during benchmark runs (see also #455). This change is somewhat similar to the Clock introduced to avoid gettimeofday() syscalls in #457.

For a single HTTP request, no syscalls can be eliminated, but for any further request over the same connection (Connection: keep-alive), these syscalls can be eliminated:

pselect6(5, [3 4], [], [], NULL, NULL)  = 1 (in [3])
poll([{fd=3, events=POLLIN|POLLPRI|POLLERR|POLLHUP}], 1, 0) = 1 ([{fd=3, revents=POLLIN}])
recvfrom(3, "G", 1, MSG_PEEK, NULL, NULL) = 1
recvfrom(3, "GET / HTTP/1.1\r\nHost: localhost:"..., 65536, 0, NULL, NULL) = 78
poll([{fd=3, events=POLLIN|POLLPRI|POLLERR|POLLHUP}], 1, 0) = 0 (Timeout)
recvfrom(3, 0x7fceeb63d066, 65458, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable)
- getpeername(3, {sa_family=AF_INET6, sin6_port=htons(54462), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "::ffff:127.0.0.1", &sin6_addr), sin6_scope_id=0}, [128 => 28]) = 0
- getsockname(3, {sa_family=AF_INET6, sin6_port=htons(8080), sin6_flowinfo=htonl(0), inet_pton(AF_INET6, "::ffff:127.0.0.1", &sin6_addr), sin6_scope_id=0}, [128 => 28]) = 0
gettimeofday({tv_sec=1652528350, tv_usec=915571}, NULL) = 0
pselect6(5, [3 4], [3], [], NULL, NULL) = 1 (out [3])
sendto(3, "HTTP/1.1 200 OK\r\nContent-Type: t"..., 150, 0, NULL, 0) = 150
pselect6(5, [3 4], [], [], NULL, NULL)  = 1 (in [3])
poll([{fd=3, events=POLLIN|POLLPRI|POLLERR|POLLHUP}], 1, 0) = 1 ([{fd=3, revents=POLLIN}])
recvfrom(3, "", 1, MSG_PEEK, NULL, NULL) = 0
recvfrom(3, "", 65536, 0, NULL, NULL)   = 0
shutdown(3, SHUT_RDWR)                  = 0
fcntl(3, F_GETFL)                       = 0x802 (flags O_RDWR|O_NONBLOCK)
fcntl(3, F_SETFL, O_RDWR|O_NONBLOCK)    = 0
close(3)                                = 0

pselect6(5, [4], [], [], NULL, NULL)    = …

This is especially noticeable during a benchmark run. In particular, the HTTP server may handle any number of requests over a single connection, so the socket addresses will only be checked once and reused for all following requests. Running docker run -it --rm --net=host jordi/ab -n1000000 -c50 -k http://localhost:8080/ against the example HTTP server suggests results improved from 22624 req/s to 25301 req/s on my machine (best of 5 each).

This changeset only refactors some internal logic and does not affect the public API, so it should be safe to apply. The test suite confirms this has 100% code coverage.

Together with #457, this closes #455