API: Make maps the right size from the start

[jannotti]

Should be a tiny gain. It bothered me that we knew the size and didn't use it.

Summary

Test Plan

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (d383a91) 55.96% compared to head (664aede) 55.97%.

Files	Patch %	Lines
data/pools/statusCache.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5906      +/-   ##
==========================================
+ Coverage   55.96%   55.97%   +0.01%     
==========================================
  Files         477      477              
  Lines       67463    67463              
==========================================
+ Hits        37754    37764      +10     
+ Misses      27154    27150       -4     
+ Partials     2555     2549       -6     

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[algorandskiy]

so rationale here is the following: since the cache is alive as long as txpool (all along) it is OK to preallocate once because it will grow to this size anyway (in a round under high load or during hundreds of round under low load).
LGTM.