feat(events): implement metadata events

[glevco]

• Unify TX_METADATA_CHANGED and BLOCK_METADATA_CHANGED events into VERTEX_METADATA_CHANGED
• Implement ability on EventManager to convert an event to another type before persisting and emitting it

[codecov]

Codecov Report

Merging #530 (5b2a5c2) into dev (1243e51) will decrease coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #530      +/-   ##
==========================================
- Coverage   83.17%   83.12%   -0.05%     
==========================================
  Files         209      209              
  Lines       18512    18512              
  Branches     2587     2587              
==========================================
- Hits        15397    15389       -8     
- Misses       2554     2558       +4     
- Partials      561      565       +4     

Impacted Files	Coverage Δ
hathor/event/event_manager.py	94.93% <100.00%> (+0.06%)	⬆️
hathor/pubsub.py	94.93% <100.00%> (-0.07%)	⬇️
hathor/consensus.py	95.79% <0.00%> (-1.10%)	⬇️
hathor/manager.py	70.08% <0.00%> (-0.15%)	⬇️
hathor/transaction/storage/transaction_storage.py	89.97% <0.00%> (ø)

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