feat: add get-best-blockchain capability

[alexruzenhack]

Acceptance criteria

• Add capability GET-BEST-BLOCKCHAIN
• Make the new capability default
• Add capabilities to HathorProtocol in HelloState
• Implement commands GET-BEST-BLOCKCHAIN and BEST-BLOCKCHAIN in ReadyState
• Test all together

Closes:

• Add GET-BEST-BLOCKCHAIN capability #677
• Implement GET-BEST-BLOCKCHAIN in the ReadyState #678
• Test GET-BEST-BLOCKCHAIN capability #679

[codecov]

Codecov Report

Merging #686 (9ab9d43) into master (e710e6d) will increase coverage by 0.15%.
Report is 1 commits behind head on master.
The diff coverage is 91.35%.

@@            Coverage Diff             @@
##           master     #686      +/-   ##
==========================================
+ Coverage   84.43%   84.59%   +0.15%     
==========================================
  Files         251      252       +1     
  Lines       21607    21686      +79     
  Branches     2923     2936      +13     
==========================================
+ Hits        18244    18345     +101     
+ Misses       2724     2707      -17     
+ Partials      639      634       -5     

Files Changed	Coverage Δ
hathor/transaction/block.py	95.35% <ø> (ø)
hathor/p2p/states/utils.py	62.50% <62.50%> (ø)
hathor/p2p/states/ready.py	93.93% <97.50%> (+1.54%)	⬆️
hathor/conf/settings.py	97.16% <100.00%> (+0.08%)	⬆️
hathor/manager.py	82.68% <100.00%> (ø)
hathor/p2p/messages.py	100.00% <100.00%> (ø)
hathor/p2p/protocol.py	93.04% <100.00%> (+0.05%)	⬆️
hathor/p2p/states/hello.py	72.00% <100.00%> (+0.28%)	⬆️
hathor/transaction/storage/transaction_storage.py	92.86% <100.00%> (+0.26%)	⬆️
hathor/types.py	100.00% <100.00%> (ø)

... and 6 files with indirect coverage changes