network: tiny speedup, less allocation

[jannotti]

Very small twekas while reading txHandler.go

This makes the saltedCache a tiny bit faster, and perform fewer allocs.

                                                  │ before.txt  │             after.txt              │
                                                  │   sec/op    │   sec/op     vs base               │
DigestCaches/data.digestCacheMaker/threads=1-10     720.7n ± 1%   723.9n ± 1%       ~ (p=0.210 n=10)
DigestCaches/data.saltedCacheMaker/threads=1-10     988.0n ± 1%   979.1n ± 1%  -0.90% (p=0.015 n=10)
DigestCaches/data.digestCacheMaker/threads=4-10     764.9n ± 1%   763.1n ± 1%       ~ (p=0.403 n=10)
DigestCaches/data.saltedCacheMaker/threads=4-10     1.319µ ± 0%   1.309µ ± 0%  -0.80% (p=0.000 n=10)
DigestCaches/data.digestCacheMaker/threads=16-10    997.5n ± 1%   994.1n ± 1%       ~ (p=0.305 n=10)
DigestCaches/data.saltedCacheMaker/threads=16-10    1.325µ ± 0%   1.309µ ± 1%  -1.21% (p=0.002 n=10)
DigestCaches/data.digestCacheMaker/threads=128-10   1.023µ ± 1%   1.020µ ± 0%       ~ (p=0.147 n=10)
DigestCaches/data.saltedCacheMaker/threads=128-10   1.356µ ± 0%   1.336µ ± 0%  -1.47% (p=0.000 n=10)
geomean                                             1.035µ        1.029µ       -0.61%

                                                  │ before.txt  │              after.txt               │
                                                  │    B/op     │    B/op     vs base                  │
DigestCaches/data.digestCacheMaker/threads=1-10     173.0 ±  2%   173.5 ± 0%        ~ (p=0.331 n=10)
DigestCaches/data.saltedCacheMaker/threads=1-10     171.0 ±  1%   138.0 ± 1%  -19.30% (p=0.000 n=10)
DigestCaches/data.digestCacheMaker/threads=4-10     184.0 ±  1%   184.0 ± 1%        ~ (p=0.914 n=10)
DigestCaches/data.saltedCacheMaker/threads=4-10     189.5 ±  1%   156.0 ± 1%  -17.68% (p=0.000 n=10)
DigestCaches/data.digestCacheMaker/threads=16-10    140.0 ± 11%   139.0 ± 1%        ~ (p=0.138 n=10)
DigestCaches/data.saltedCacheMaker/threads=16-10    189.5 ±  1%   156.0 ± 1%  -17.68% (p=0.000 n=10)
DigestCaches/data.digestCacheMaker/threads=128-10   156.0 ±  0%   156.0 ± 0%        ~ (p=1.000 n=10) ¹
DigestCaches/data.saltedCacheMaker/threads=128-10   193.5 ±  1%   159.0 ± 1%  -17.83% (p=0.000 n=10)
geomean                                             173.6         157.0        -9.56%
¹ all samples are equal

                                                  │ before.txt │              after.txt               │
                                                  │ allocs/op  │ allocs/op   vs base                  │
DigestCaches/data.digestCacheMaker/threads=1-10     1.000 ± 0%   1.000 ± 0%        ~ (p=1.000 n=10) ¹
DigestCaches/data.saltedCacheMaker/threads=1-10     2.000 ± 0%   1.000 ± 0%  -50.00% (p=0.000 n=10)
DigestCaches/data.digestCacheMaker/threads=4-10     1.000 ± 0%   1.000 ± 0%        ~ (p=1.000 n=10) ¹
DigestCaches/data.saltedCacheMaker/threads=4-10     2.000 ± 0%   1.000 ± 0%  -50.00% (p=0.000 n=10)
DigestCaches/data.digestCacheMaker/threads=16-10    1.000 ± 0%   1.000 ± 0%        ~ (p=1.000 n=10) ¹
DigestCaches/data.saltedCacheMaker/threads=16-10    2.000 ± 0%   1.000 ± 0%  -50.00% (p=0.000 n=10)
DigestCaches/data.digestCacheMaker/threads=128-10   1.000 ± 0%   1.000 ± 0%        ~ (p=1.000 n=10) ¹
DigestCaches/data.saltedCacheMaker/threads=128-10   2.000 ± 0%   1.000 ± 0%  -50.00% (p=0.000 n=10)
geomean                                             1.414        1.000       -29.29%

Summary

Test Plan

[codecov]

Codecov Report

Attention: Patch coverage is 89.18919% with 4 lines in your changes missing coverage. Please review.

Project coverage is 51.78%. Comparing base (5bffa0e) to head (2eea1ec).
Report is 12 commits behind head on master.

Files with missing lines	Patch %	Lines
data/txHandler.go	84.21%	2 Missing and 1 partial ⚠️
data/txDupCache.go	94.44%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6246      +/-   ##
==========================================
+ Coverage   51.76%   51.78%   +0.01%     
==========================================
  Files         644      644              
  Lines       86523    86517       -6     
==========================================
+ Hits        44790    44799       +9     
+ Misses      38864    38852      -12     
+ Partials     2869     2866       -3     

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[algorandskiy]

*crypto.Digest to crypto.Digest looks surprising because that pointer's purpose in decreasing copying and allocations in early version of golang.