Avoid growbuffer, take two

[jech]

Take two of #77. Due to overhead having been removed elsewhere, the results are much more dramatic, especially for GCM.

When I first submitted that, @Sean-Der noticed that this relies on nextConn.write not retaining a reference to the buffer. This remains true of this version. However, relying on write methods not retaining the buffer is standard practice in the Go world, as mentioned here: https://pkg.go.dev/io#Writer. I remain naturally open to other approaches, but I think that the performance improvement is too large to leave on the table, especially on embedded systems with slow memory interconnects (where the results are even more dramatic, as pointed out by @adriancable).

This depends on #196.

name                 old time/op    new time/op    delta
WriteRTP/CTR-100-8      951ns ± 0%     934ns ± 0%   -1.76%  (p=0.029 n=4+4)
WriteRTP/CTR-1400-8    4.64µs ± 0%    4.52µs ± 0%   -2.59%  (p=0.029 n=4+4)
WriteRTP/GCM-100-8      305ns ± 6%     259ns ± 0%  -15.07%  (p=0.029 n=4+4)
WriteRTP/GCM-1000-8     704ns ±12%     460ns ± 1%  -34.72%  (p=0.029 n=4+4)

name                 old speed      new speed      delta
WriteRTP/CTR-100-8    118MB/s ± 0%   120MB/s ± 0%   +1.80%  (p=0.029 n=4+4)
WriteRTP/CTR-1400-8   218MB/s ± 0%   224MB/s ± 0%   +2.65%  (p=0.029 n=4+4)
WriteRTP/GCM-100-8    368MB/s ± 6%   433MB/s ± 0%  +17.54%  (p=0.029 n=4+4)
WriteRTP/GCM-1000-8  1.45GB/s ±11%  2.20GB/s ± 1%  +51.73%  (p=0.029 n=4+4)

name                 old alloc/op   new alloc/op   delta
WriteRTP/CTR-100-8       188B ± 0%       60B ± 0%  -68.09%  (p=0.029 n=4+4)
WriteRTP/CTR-1400-8    1.08kB ± 0%    0.06kB ± 0%  -94.46%  (p=0.029 n=4+4)
WriteRTP/GCM-100-8       144B ± 0%       16B ± 0%  -88.89%  (p=0.029 n=4+4)
WriteRTP/GCM-1000-8    1.17kB ± 0%    0.02kB ± 0%  -98.63%  (p=0.029 n=4+4)

name                 old allocs/op  new allocs/op  delta
WriteRTP/CTR-100-8       5.00 ± 0%      4.00 ± 0%  -20.00%  (p=0.029 n=4+4)
WriteRTP/CTR-1400-8      5.00 ± 0%      4.00 ± 0%  -20.00%  (p=0.029 n=4+4)
WriteRTP/GCM-100-8       2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.029 n=4+4)
WriteRTP/GCM-1000-8      2.00 ± 0%      1.00 ± 0%  -50.00%  (p=0.029 n=4+4)

[codecov]

Codecov Report

Merging #197 (704fd0a) into master (16044eb) will increase coverage by 0.18%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #197      +/-   ##
==========================================
+ Coverage   74.97%   75.16%   +0.18%     
==========================================
  Files          17       17              
  Lines        1215     1224       +9     
==========================================
+ Hits          911      920       +9     
  Misses        208      208              
  Partials       96       96              

Flag	Coverage Δ
go	75.16% <100.00%> (+0.18%)	⬆️
wasm	74.67% <100.00%> (+0.18%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
session_srtcp.go	76.85% <100.00%> (+0.58%)	⬆️
session_srtp.go	74.16% <100.00%> (+1.35%)	⬆️

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[jech]

@adriancable on an in-order ARMv7 with no hardware crypto (AM335x, Cortex-A8 at 1GHz) the results are less dramatic (due to the time spent in crypto) but still siginificant:

name               old time/op    new time/op    delta
WriteRTP/CTR-100     29.3µs ± 1%    28.6µs ± 1%  -2.31%  (p=0.029 n=4+4)
WriteRTP/CTR-1400     150µs ± 0%     139µs ± 1%  -7.41%  (p=0.029 n=4+4)
WriteRTP/GCM-100     34.5µs ± 0%    33.1µs ± 1%  -3.93%  (p=0.029 n=4+4)
WriteRTP/GCM-1000     215µs ± 1%     200µs ± 0%  -6.67%  (p=0.029 n=4+4)

name               old speed      new speed      delta
WriteRTP/CTR-100   3.82MB/s ± 1%  3.92MB/s ± 1%  +2.42%  (p=0.029 n=4+4)
WriteRTP/CTR-1400  6.73MB/s ± 0%  7.27MB/s ± 1%  +7.95%  (p=0.029 n=4+4)
WriteRTP/GCM-100   3.25MB/s ± 0%  3.38MB/s ± 1%  +4.08%  (p=0.029 n=4+4)
WriteRTP/GCM-1000  4.71MB/s ± 1%  5.05MB/s ± 0%  +7.21%  (p=0.029 n=4+4)

[adriancable]

I've been running these changes for a few days on ARMv7 and x86 with no issues. The benchmark improvements are impressive. A few % shouldn't be left on the table.