catchpoint: fix peer ranking

[algorandskiy]

Summary

• Similar to tests: fix TestBasicCatchpointCatchup #4390, if the local ledger has a block, do not rank peer.
• In order to implement a unit test, had to abstract ledger.Ledger as an interface.
• Fixed some typos in comments and variable names.

Test Plan

Added a new unit test.

[codecov]

Codecov Report

Merging #4535 (661764e) into master (22b6101) will increase coverage by 0.04%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #4535      +/-   ##
==========================================
+ Coverage   54.06%   54.10%   +0.04%     
==========================================
  Files         401      401              
  Lines       51540    51547       +7     
==========================================
+ Hits        27863    27888      +25     
+ Misses      21346    21316      -30     
- Partials     2331     2343      +12     

Impacted Files	Coverage Δ
catchup/catchpointService.go	7.80% <0.00%> (+6.03%)	⬆️
catchup/ledgerFetcher.go	40.22% <ø> (ø)
ledger/catchupaccessor.go	62.03% <0.00%> (-0.13%)	⬇️
node/node.go	4.40% <0.00%> (-0.03%)	⬇️
ledger/blockqueue.go	85.63% <0.00%> (-2.88%)	⬇️
ledger/acctupdates.go	69.59% <0.00%> (-0.60%)	⬇️
network/wsNetwork.go	64.63% <0.00%> (-0.20%)	⬇️
network/wsPeer.go	68.73% <0.00%> (+0.53%)	⬆️
catchup/service.go	69.38% <0.00%> (+0.74%)	⬆️
cmd/tealdbg/debugger.go	73.49% <0.00%> (+0.80%)	⬆️
... and 3 more

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[ghost]

Looks good

[algonautshant]

Looks great.
Just one more cleanup suggestion.

[algonautshant]

Since we are cleaning up this code, will be nice to also get rid of the ledger field in CatchpointCatchupService, and rely on ledgerAccessor.Ledger() instead.