feat(events): handle events during the load phase

[glevco]

Acceptance Criteria:

• Emit LOAD_FINISHED event when the full node ends the LOAD phase.
• Remove LOAD_STARTED event, because it wasn't used and is not necessary. If needed, existing MANAGER_ON_START event serves the same purpose.
• Emit NETWORK_NEW_TX_ACCEPTED events for all txs during the LOAD phase.
• Implement --x-emit-load-events flag, that enables emission of events published during the LOAD phase.
  • It cannot be used without the --x-enable-event-queue flag.
  • It cannot be used with the --x-full-verification flag.

[codecov]

Codecov Report

Merging #534 (efe6f7d) into dev (adc0ac5) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head efe6f7d differs from pull request most recent head 9d8a616. Consider uploading reports for the commit 9d8a616 to get more accurate results

@@            Coverage Diff             @@
##              dev     #534      +/-   ##
==========================================
- Coverage   83.20%   83.20%   -0.01%     
==========================================
  Files         211      211              
  Lines       18646    18644       -2     
  Branches     2590     2590              
==========================================
- Hits        15514    15512       -2     
+ Misses       2563     2562       -1     
- Partials      569      570       +1     

Impacted Files	Coverage Δ
hathor/event/event_manager.py	97.01% <ø> (ø)
hathor/event/model/base_event.py	92.30% <ø> (ø)
hathor/pubsub.py	94.80% <ø> (-0.14%)	⬇️

... and 3 files with indirect coverage changes

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[jansegre]

For me it's approved considering that we discussed to leave this provisional and make tweaks later.