refactor(events): improve events during the load phase

[glevco]

Acceptance Criteria

• Change handling of events during the load phase, so it's not async anymore
  • Remove HathorEvents.LOAD_FINISHED as it's no used anymore
• Add progress tracker for handling of load phase events
• Implement EventStorage.save_events()

[codecov]

Codecov Report

Merging #652 (12c7d64) into master (1d5227c) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #652      +/-   ##
==========================================
+ Coverage   84.09%   84.11%   +0.01%     
==========================================
  Files         246      246              
  Lines       20418    20442      +24     
  Branches     2754     2757       +3     
==========================================
+ Hits        17170    17194      +24     
  Misses       2674     2674              
  Partials      574      574              

Impacted Files	Coverage Δ
hathor/event/model/event_type.py	100.00% <ø> (ø)
hathor/pubsub.py	95.94% <ø> (-0.06%)	⬇️
hathor/event/event_manager.py	91.66% <100.00%> (+0.84%)	⬆️
hathor/event/storage/event_storage.py	100.00% <100.00%> (ø)
hathor/event/storage/memory_storage.py	100.00% <100.00%> (ø)
hathor/event/storage/rocksdb_storage.py	94.68% <100.00%> (+0.49%)	⬆️
hathor/manager.py	77.25% <100.00%> (+0.03%)	⬆️