Check the validity of ECChain first in chain exchange

[masih]

Add ECChain validation to chain exchange pubsub validation function to reject invalid chains as early as possible.

Change the ordering of validation rules such that cheaper checks are done first before checking current progress.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Project coverage is 67.32%. Comparing base (9c11ba3) to head (6ca8dc2).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
chainexchange/pubsub.go	0.00%	7 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #837      +/-   ##
==========================================
- Coverage   67.39%   67.32%   -0.07%     
==========================================
  Files          84       84              
  Lines        8915     8921       +6     
==========================================
- Hits         6008     6006       -2     
- Misses       2380     2388       +8     
  Partials      527      527              

Files with missing lines	Coverage Δ
chainexchange/pubsub.go	74.78% <0.00%> (-3.29%)	⬇️

... and 5 files with indirect coverage changes