txHandler: kick in ARL at 1/2 of a base backlog capacity

[algorandskiy]

Summary

While checking ARL after deployment I found out ARL can be more aggressive. The original settings kicked in it at 1/2 of the adjusted to ERL capacity (defaults are 26k + 2.4k*20 = 74k, so it kicks in at 36k backlogged messages).
Logs showed these settings are fine for extreme traffic with full txn groups but not enough for high traffic where the backlog stays at ~20k messages. Given the fact 20k messages is kind of enough to make almost full blocks, we have not observed committed 20k txns per block during these time periods. So that most of these txns are junk and can be discarded earlier.

Test Plan

Adjusted unit test to ensure the new kick-in condition.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (8f8ada5) 55.91% compared to head (5b3d29d) 55.98%.

Files	Patch %	Lines
data/txHandler.go	62.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5873      +/-   ##
==========================================
+ Coverage   55.91%   55.98%   +0.07%     
==========================================
  Files         477      477              
  Lines       67407    67408       +1     
==========================================
+ Hits        37689    37740      +51     
+ Misses      27157    27118      -39     
+ Partials     2561     2550      -11     

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