fix(event-queue): improve memory usage during the load phase

[glevco]

Motivation

The previous code was consuming too much memory when loading events from an existing database.

Acceptance Criteria

• Create new batch_iterator() utils function that converts an iterator of items into an iterator of batches.
• Change EventManager.handle_load_phase_vertices() so it saves events to RocksDB in write batches of 10.000 events instead of a single batch with all events.

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

Merging #744 (2fb77fc) into master (c95834d) will decrease coverage by 0.05%.
Report is 2 commits behind head on master.
The diff coverage is 100.00%.

❗ Current head 2fb77fc differs from pull request most recent head a909ab2. Consider uploading reports for the commit a909ab2 to get more accurate results

@@            Coverage Diff             @@
##           master     #744      +/-   ##
==========================================
- Coverage   84.56%   84.51%   -0.05%     
==========================================
  Files         252      253       +1     
  Lines       21688    21705      +17     
  Branches     2937     2941       +4     
==========================================
+ Hits        18341    18345       +4     
- Misses       2709     2719      +10     
- Partials      638      641       +3     

Files Changed	Coverage Δ
hathor/event/event_manager.py	92.00% <100.00%> (+0.33%)	⬆️
hathor/utils/iter.py	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes