net.TCPConn has the ability to send data out using system calls such as
sendfile when the source data comes from an *os.File. However, the way
that I/O has been laid out in the transport means that the File is
actually wrapped behind two outer io.Readers, and as such the TCP stack
cannot properly type-assert the reader, ensuring that it falls back to
genericReadFrom.

This commit does the following:

* Removes transferBodyReader and moves its functionality to a new
doBodyCopy helper. This is not an io.Reader implementation, but no
functionality is lost this way, and it allows us to unwrap one layer
from the body.

* The second layer of the body is unwrapped if the original reader
was wrapped with ioutil.NopCloser, which is what NewRequest wraps the
body in if it's not a ReadCloser on its own. The unwrap operation
passes through the existing body if there's no nopCloser.

Note that this depends on change https://golang.org/cl/163737 to
properly function, as the lack of ReaderFrom implementation otherwise
means that this functionality is essentially walled off.

Benchmarks between this commit and https://golang.org/cl/163862,
incorporating https://golang.org/cl/163737:

linux/amd64:
name                        old time/op    new time/op    delta
FileAndServer_1KB/NoTLS-4     53.2µs ± 0%    53.3µs ± 0%      ~     (p=0.075 n=10+9)
FileAndServer_1KB/TLS-4       61.2µs ± 0%    60.7µs ± 0%    -0.77%  (p=0.000 n=10+9)
FileAndServer_16MB/NoTLS-4    25.3ms ± 5%     3.8ms ± 6%   -84.95%  (p=0.000 n=10+10)
FileAndServer_16MB/TLS-4      33.2ms ± 2%    13.4ms ± 2%   -59.57%  (p=0.000 n=10+10)
FileAndServer_64MB/NoTLS-4     106ms ± 4%      16ms ± 2%   -84.45%  (p=0.000 n=10+10)
FileAndServer_64MB/TLS-4       129ms ± 1%      54ms ± 3%   -58.32%  (p=0.000 n=8+10)

name                        old speed      new speed      delta
FileAndServer_1KB/NoTLS-4   19.2MB/s ± 0%  19.2MB/s ± 0%      ~     (p=0.095 n=10+9)
FileAndServer_1KB/TLS-4     16.7MB/s ± 0%  16.9MB/s ± 0%    +0.78%  (p=0.000 n=10+9)
FileAndServer_16MB/NoTLS-4   664MB/s ± 5%  4415MB/s ± 6%  +565.27%  (p=0.000 n=10+10)
FileAndServer_16MB/TLS-4     505MB/s ± 2%  1250MB/s ± 2%  +147.32%  (p=0.000 n=10+10)
FileAndServer_64MB/NoTLS-4   636MB/s ± 4%  4090MB/s ± 2%  +542.81%  (p=0.000 n=10+10)
FileAndServer_64MB/TLS-4     522MB/s ± 1%  1251MB/s ± 3%  +139.95%  (p=0.000 n=8+10)

darwin/amd64:
name                        old time/op    new time/op     delta
FileAndServer_1KB/NoTLS-8     93.0µs ± 5%     96.6µs ±11%      ~     (p=0.190 n=10+10)
FileAndServer_1KB/TLS-8        105µs ± 7%      100µs ± 5%    -5.14%  (p=0.002 n=10+9)
FileAndServer_16MB/NoTLS-8    87.5ms ±19%     10.0ms ± 6%   -88.57%  (p=0.000 n=10+10)
FileAndServer_16MB/TLS-8      52.7ms ±11%     17.4ms ± 5%   -66.92%  (p=0.000 n=10+10)
FileAndServer_64MB/NoTLS-8     363ms ±54%       39ms ± 7%   -89.24%  (p=0.000 n=10+10)
FileAndServer_64MB/TLS-8       209ms ±13%       73ms ± 5%   -65.37%  (p=0.000 n=9+10)

name                        old speed      new speed       delta
FileAndServer_1KB/NoTLS-8   11.0MB/s ± 5%   10.6MB/s ±10%      ~     (p=0.184 n=10+10)
FileAndServer_1KB/TLS-8     9.75MB/s ± 7%  10.27MB/s ± 5%    +5.26%  (p=0.003 n=10+9)
FileAndServer_16MB/NoTLS-8   194MB/s ±16%   1680MB/s ± 6%  +767.83%  (p=0.000 n=10+10)
FileAndServer_16MB/TLS-8     319MB/s ±10%    963MB/s ± 4%  +201.36%  (p=0.000 n=10+10)
FileAndServer_64MB/NoTLS-8   180MB/s ±31%   1719MB/s ± 7%  +853.61%  (p=0.000 n=9+10)
FileAndServer_64MB/TLS-8     321MB/s ±12%    926MB/s ± 5%  +188.24%  (p=0.000 n=9+10)

Updates #30377.

Change-Id: I631a73cea75371dfbb418c9cd487c4aa35e73fcd
GitHub-Last-Rev: 4a77dd1b80140274bf3ed20ad7465ff3cc06febf
GitHub-Pull-Request: golang/go#30378
Reviewed-on: https://go-review.googlesource.com/c/go/+/163599
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>