network: make txBacklogSize responsive to block size

[brianolson]

Summary

Make a channel length txBacklogSize responsive to up coming changes in block size. Hold approximately one block of txns in channel. Manual testing has shown that txBacklogSize needs to be increased from its original size of 1000.

Test Plan

Manually experimented and verified in cluster tests.

[codecov]

Codecov Report

Merging #4377 (8549fb7) into master (60fb4f6) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4377      +/-   ##
==========================================
- Coverage   55.60%   55.59%   -0.01%     
==========================================
  Files         403      403              
  Lines       50801    50801              
==========================================
- Hits        28248    28244       -4     
- Misses      20159    20162       +3     
- Partials     2394     2395       +1     

Impacted Files	Coverage Δ
data/txHandler.go	0.00% <ø> (ø)
agreement/cryptoVerifier.go	67.60% <0.00%> (-2.12%)	⬇️
agreement/proposalManager.go	96.07% <0.00%> (-1.97%)	⬇️
catchup/peerSelector.go	98.95% <0.00%> (-1.05%)	⬇️
ledger/acctupdates.go	70.37% <0.00%> (-0.16%)	⬇️
ledger/tracker.go	78.63% <0.00%> (ø)
catchup/service.go	69.38% <0.00%> (ø)
data/abi/abi_type.go	88.62% <0.00%> (+0.94%)	⬆️
util/db/dbutil.go	50.30% <0.00%> (+1.21%)	⬆️

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