Use bitcoinheaders.net v2 format

[t-bast]

The format used by bitcoinheaders.net is changing to use whole bytes instead of nibles, which is easier to parse. We start using the v2 format exclusively, which will allow deprecating the previous format.

Thanks @TheBlueMatt for notifying us.

Fixes #2786

[codecov-commenter]

Codecov Report

Merging #2787 (886f09c) into master (e73c1cf) will increase coverage by 0.06%.
The diff coverage is 0.00%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2787      +/-   ##
==========================================
+ Coverage   85.83%   85.90%   +0.06%     
==========================================
  Files         216      216              
  Lines       18169    18166       -3     
  Branches      753      756       +3     
==========================================
+ Hits        15596    15606      +10     
+ Misses       2573     2560      -13     

Files	Coverage Δ
...q/eclair/blockchain/watchdogs/HeadersOverDns.scala	17.77% <0.00%> (+1.11%)	⬆️

... and 5 files with indirect coverage changes

[remyers]

I reviewed the changes and stepped through the tests to make sure I understood what it was doing. Looks good to me.