fix: ensure header.Validate is called by the lib

[Wondertan]

Header implementations should not call Validate themselves as the lib now controls it.

Additionally, we unify the response processing code and Header constructor.

Closes #78 Based on #88

[codecov-commenter]

Codecov Report

Merging #89 (c51aa1e) into main (e500905) will decrease coverage by 0.08%. The diff coverage is 67.74%.

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
- Coverage   68.34%   68.27%   -0.08%     
==========================================
  Files          36       37       +1     
  Lines        2903     2909       +6     
==========================================
+ Hits         1984     1986       +2     
- Misses        768      771       +3     
- Partials      151      152       +1     

Files Changed	Coverage Δ
p2p/subscriber.go	44.64% <45.45%> (-5.36%)	:arrow_down:
p2p/exchange.go	85.65% <62.50%> (+1.97%)	:arrow_up:
p2p/session.go	91.23% <70.96%> (-1.24%)	:arrow_down:
store/store.go	67.91% <75.00%> (-0.11%)	:arrow_down:
header.go	100.00% <100.00%> (ø)
store/init.go	54.54% <100.00%> (ø)

... and 2 files with indirect coverage changes