It used to be simple, and then it got complicated for speed (to reduce
allocations, mostly), but that involved a mutex and hurt multi-core
performance, contending on the mutex.

A change was sent to try to improve that mutex contention in
https://go-review.googlesource.com/c/42110/2/src/net/http/server.go
but that introduced its own allocations (the string->interface{}
boxing for the sync.Map key), which runs counter to the whole point of
that statusLine function: to remove allocations.

Instead, make the code simple again and not have a mutex. It's a bit
slower for the single-core case, but nobody with a single-user HTTP
server cares about 50 nanoseconds:

name                  old time/op    new time/op    delta
ResponseStatusLine      37.5ns ± 2%    87.1ns ± 2%  +132.42%          (p=0.029 n=4+4)
ResponseStatusLine-2    63.1ns ± 1%    43.1ns ±12%   -31.67%          (p=0.029 n=4+4)
ResponseStatusLine-4    53.8ns ± 8%    40.2ns ± 2%   -25.29%          (p=0.029 n=4+4)

name                  old alloc/op   new alloc/op   delta
ResponseStatusLine      0.00B ±NaN%    0.00B ±NaN%      ~     (all samples are equal)
ResponseStatusLine-2    0.00B ±NaN%    0.00B ±NaN%      ~     (all samples are equal)
ResponseStatusLine-4    0.00B ±NaN%    0.00B ±NaN%      ~     (all samples are equal)

name                  old allocs/op  new allocs/op  delta
ResponseStatusLine       0.00 ±NaN%     0.00 ±NaN%      ~     (all samples are equal)
ResponseStatusLine-2     0.00 ±NaN%     0.00 ±NaN%      ~     (all samples are equal)
ResponseStatusLine-4     0.00 ±NaN%     0.00 ±NaN%      ~     (all samples are equal)

(Note the code could be even simpler with fmt.Fprintf, but that is
 relatively slow and involves a bunch of allocations getting arguments
 into interface{} for the call)

Change-Id: I1fa119132dbbf97a8e7204ce3e0707d433060da2
Reviewed-on: https://go-review.googlesource.com/42133
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>