net/http: reduce memory usage when hijacking

[das7pad]

Previously, Hijack allocated a new write buffer and the existing
connection write buffer used an extra 4kB of memory until the handler
finished and the "conn" was garbage collected. Now, hijack re-uses the
existing write buffer and re-attaches it to the raw connection to avoid
referencing the net/http "conn" after returning.

Hijack will also swap the underlying reader of the read buffer to avoid
holding on to the "connReader", which in turn holds on to the "conn".
This allows all of the "conn", "response" and "Request" to get garbage
collected after the handler has exited.

The new underlying reader of the read buffer uses a weak-ref on the
"response" to allow CloseNotify to work before the handler has exited
(i.e. while the "response" is still referenced in "conn.serve"). This
aligns with the documentation of CloseNotifier:

After the Handler has returned, there is no guarantee that the channel
receives a value.

It is possible to preserve the extended behavior of letting CloseNotify
signal after the handler has exited by keeping around the relevant state
for closeNotify in a struct that is used in both "connReader" and the
new "connCloseNotify". It does not seem worth the extra code complexity
for the deprecated CloseNotifier interface.

To achieve a net-positive effect on allocations per request cycle, the
TODO for lazily allocating the CloseNotifier channel has been addressed.
This change also simplifies the read error handling in the new
"connCloseNotify" wrapper.

goos: linux
goarch: amd64
pkg: net/http
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
│ before │ writer │ lazy-notify │ reader │
│ sec/op │ sec/op vs base │ sec/op vs base │ sec/op vs base │
Server-8 506.6µ ± 1% 508.4µ ± 2% ~ (p=0.315 n=10) 507.0µ ± 1% ~ (p=0.796 n=10) 499.5µ ± 2% -1.40% (p=0.004 n=10)
ServerHijack-8 50.64µ ± 5% 46.19µ ± 5% -8.79% (p=0.000 n=10) 44.09µ ± 29% -12.94% (p=0.000 n=10) 44.56µ ± 27% -12.01% (p=0.002 n=10)
ServerHijackMemoryUsage-8 1.078m ± 3% 1.112m ± 5% ~ (p=0.436 n=10) 1.181m ± 15% ~ (p=0.190 n=10) 1.104m ± 9% ~ (p=0.190 n=10)
geomean 302.4µ 296.7µ -1.89% 297.7µ -1.54% 290.7µ -3.86%

                      │    before    │                writer                │             lazy-notify              │                reader                │
                      │     B/op     │     B/op      vs base                │     B/op      vs base                │     B/op      vs base                │

Server-8 2.298Ki ± 0% 2.297Ki ± 0% ~ (p=0.781 n=10) 2.185Ki ± 0% -4.91% (p=0.000 n=10) 2.188Ki ± 0% -4.78% (p=0.000 n=10)
ServerHijack-8 16.21Ki ± 0% 12.14Ki ± 0% -25.11% (p=0.000 n=10) 12.03Ki ± 0% -25.78% (p=0.000 n=10) 12.09Ki ± 0% -25.45% (p=0.000 n=10)
ServerHijackMemoryUsage-8 15.68Ki ± 0% 11.61Ki ± 0% -25.91% (p=0.000 n=10) 11.51Ki ± 0% -26.58% (p=0.000 n=10) 11.57Ki ± 0% -26.21% (p=0.000 n=10)
geomean 8.358Ki 6.867Ki -17.84% 6.714Ki -19.68% 6.738Ki -19.39%

                      │   before   │               writer                │            lazy-notify            │              reader               │
                      │ allocs/op  │ allocs/op   vs base                 │ allocs/op   vs base               │ allocs/op   vs base               │

Server-8 21.00 ± 0% 21.00 ± 0% ~ (p=1.000 n=10) ¹ 20.00 ± 0% -4.76% (p=0.000 n=10) 20.00 ± 0% -4.76% (p=0.000 n=10)
ServerHijack-8 52.00 ± 0% 50.00 ± 0% -3.85% (p=0.000 n=10) 49.00 ± 0% -5.77% (p=0.000 n=10) 51.00 ± 0% -1.92% (p=0.000 n=10)
ServerHijackMemoryUsage-8 48.00 ± 0% 46.00 ± 0% -4.17% (p=0.000 n=10) 45.00 ± 0% -6.25% (p=0.000 n=10) 47.00 ± 0% -2.08% (p=0.000 n=10)
geomean 37.42 36.42 -2.69% 35.33 -5.60% 36.33 -2.93%
¹ all samples are equal

                      │    before     │                writer                 │              lazy-notify              │                reader                 │
                      │ retained_B/op │ retained_B/op  vs base                │ retained_B/op  vs base                │ retained_B/op  vs base                │

ServerHijackMemoryUsage-8 15.625k ± 0% 11.464k ± 0% -26.63% (p=0.000 n=10) 11.339k ± 0% -27.43% (p=0.000 n=10) 8.714k ± 0% -44.23% (p=0.000 n=10)

Fixes #59567

[das7pad]

It is possible to preserve the extended behavior of letting CloseNotify
signal after the handler has exited by keeping around the relevant state
for closeNotify in a struct that is used in both "connReader" and the
new "connCloseNotify". It does not seem worth the extra code complexity
for the deprecated CloseNotifier interface.

2bc45c1 is a WIP implementation for this -- in case this behavior is deemed useful to keep/worth the complexity.

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