accountdb: add migration 8 for resetting accounthashes on betanet

[cce]

Summary

Following on #4804 and #4812, this migration rebuilds the accounthashes table for betanet nodes.

Test Plan

Existing tests should pass, manual tests can confirm table is rebuilt.

[codecov]

Codecov Report

Merging #4822 (b3073dd) into master (d80de1a) will decrease coverage by 0.02%.
The diff coverage is 40.00%.

@@            Coverage Diff             @@
##           master    #4822      +/-   ##
==========================================
- Coverage   54.67%   54.65%   -0.03%     
==========================================
  Files         417      417              
  Lines       53734    53749      +15     
==========================================
- Hits        29381    29377       -4     
- Misses      21920    21939      +19     
  Partials     2433     2433              

Impacted Files	Coverage Δ
ledger/accountdb.go	72.35% <ø> (ø)
ledger/catchupaccessor.go	63.60% <0.00%> (-0.24%)	⬇️
ledger/trackerdb.go	46.64% <46.15%> (-0.03%)	⬇️
ledger/roundlru.go	90.56% <0.00%> (-5.67%)	⬇️
ledger/tracker.go	74.89% <0.00%> (-2.98%)	⬇️
catchup/peerSelector.go	98.95% <0.00%> (-1.05%)	⬇️
catchup/service.go	68.22% <0.00%> (-0.74%)	⬇️
network/wsPeer.go	71.36% <0.00%> (-0.72%)	⬇️
ledger/acctupdates.go	69.51% <0.00%> (-0.37%)	⬇️
network/wsNetwork.go	67.17% <0.00%> (+0.17%)	⬆️
... and 3 more

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[onetechnical]

Chris manually tested this on his database, so merging after a review.