fix(sync): fix two synchronization issues

[Wondertan]

Closes: #67 The issues:

• The syncGetter usage was racy. It had working single-flight protection, but all the routines awaiting the result returned all together and set the new header, causing the Store to receive the same headers awaited routines amount of time.
• The race between headers received from gossip and requested by Head callers.

[codecov-commenter]

Codecov Report

Merging #64 (46b06c1) into main (0c1ebfb) will increase coverage by 0.03%. The diff coverage is 66.66%.

:exclamation: Current head 46b06c1 differs from pull request most recent head be2f8e5. Consider uploading reports for the commit be2f8e5 to get more accurate results

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
+ Coverage   65.57%   65.60%   +0.03%     
==========================================
  Files          35       35              
  Lines        2806     2832      +26     
==========================================
+ Hits         1840     1858      +18     
- Misses        811      817       +6     
- Partials      155      157       +2     

Impacted Files	Coverage Δ
sync/sync.go	64.32% <ø> (ø)
sync/sync_head.go	62.40% <41.17%> (+0.36%)	:arrow_up:
sync/sync_getter.go	90.47% <89.47%> (-9.53%)	:arrow_down: