feat(sync): Implement a subjectiveHead atomic pointer on the node to use

[renaynay]

Resolves #99

[codecov-commenter]

Codecov Report

Merging #108 (91d975a) into main (e7d24b8) will increase coverage by 0.15%. The diff coverage is 74.11%.

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
+ Coverage   69.09%   69.25%   +0.15%     
==========================================
  Files          37       37              
  Lines        2938     2979      +41     
==========================================
+ Hits         2030     2063      +33     
- Misses        758      768      +10     
+ Partials      150      148       -2     

Files	Coverage Δ
sync/sync_store.go	83.33% <100.00%> (+2.08%)	:arrow_up:
sync/sync_head.go	71.12% <94.11%> (+1.89%)	:arrow_up:
headertest/dummy_suite.go	87.50% <28.57%> (-12.50%)	:arrow_down:
headertest/dummy_header.go	72.58% <40.00%> (-0.28%)	:arrow_down:
sync/sync.go	73.52% <79.16%> (+0.38%)	:arrow_up:

... and 1 file with indirect coverage changes

[renaynay]

Something is borked with sync.

[renaynay]

Nvm bug was in main