ledger: turn deferredCommitContext.newBase into a function

[cce]

Summary

Based on observations from #4419 it seems like deferredCommitContext.newBase is always set to the value of oldBase + offset, and is only stored on the DCC struct for code deduplication / convenience reasons. To enforce the invariant that newBase is always oldBase+offset and guard against bugs if this were not true, this turns it into a function rather than an assigned struct field.

Test Plan

Existing tests updated and should pass.

[codecov]

Codecov Report

Merging #5093 (0082feb) into master (4e652f2) will decrease coverage by 0.01%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##           master    #5093      +/-   ##
==========================================
- Coverage   53.42%   53.41%   -0.01%     
==========================================
  Files         431      431              
  Lines       54356    54356              
==========================================
- Hits        29037    29033       -4     
- Misses      23057    23061       +4     
  Partials     2262     2262              

Impacted Files	Coverage Δ
ledger/txtail.go	78.52% <75.00%> (ø)
ledger/catchpointtracker.go	57.90% <83.33%> (ø)
ledger/acctonline.go	78.64% <100.00%> (ø)
ledger/acctupdates.go	68.99% <100.00%> (-0.25%)	⬇️
ledger/tracker.go	74.26% <100.00%> (-0.85%)	⬇️
catchup/service.go	68.94% <0.00%> (-1.42%)	⬇️
cmd/tealdbg/debugger.go	71.65% <0.00%> (-0.79%)	⬇️
ledger/testing/randomAccounts.go	56.26% <0.00%> (-0.62%)	⬇️
... and 3 more

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more