feat(traversal): add BFS order traversal

[jansegre]

Acceptance criteria

• BFSWalk should be renamed to BFSTimestampWalk to reflect that it prioritizes by timestamp
• add BFSOrderWalk which uses actual neighbor order to yield

[codecov]

Codecov Report

Merging #672 (28e2598) into master (441c255) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #672      +/-   ##
==========================================
+ Coverage   83.84%   83.89%   +0.04%     
==========================================
  Files         246      246              
  Lines       20340    20390      +50     
  Branches     2746     2749       +3     
==========================================
+ Hits        17055    17107      +52     
+ Misses       2712     2709       -3     
- Partials      573      574       +1     

Impacted Files	Coverage Δ
hathor/builder/builder.py	92.99% <100.00%> (ø)
hathor/conf/settings.py	97.05% <100.00%> (+0.02%)	⬆️
hathor/consensus/block_consensus.py	94.36% <100.00%> (ø)
hathor/consensus/transaction_consensus.py	96.11% <100.00%> (ø)
hathor/feature_activation/feature_service.py	97.22% <100.00%> (+0.34%)	⬆️
hathor/indexes/mempool_tips_index.py	92.38% <100.00%> (ø)
hathor/p2p/sync_v1/agent.py	85.01% <100.00%> (-0.58%)	⬇️
hathor/simulator/simulator.py	94.61% <100.00%> (+0.06%)	⬆️
hathor/transaction/base_transaction.py	90.62% <100.00%> (+0.01%)	⬆️
hathor/transaction/storage/transaction_storage.py	91.04% <100.00%> (-0.02%)	⬇️
... and 1 more

... and 3 files with indirect coverage changes