refactor(sync-v2): improve deferreds handling

[glevco]

Motivation

Simplify deferred-handling code by removing repetition and improving typing robustness.

Acceptance Criteria

• Remove usage of string keys for "singular" deferreds (tips, best-block, and peer-block-hashes), by using a specific, typed attribute for each of them, instead of storing them in a generic dictionary.
• By doing that, the deferreds dictionary is now only used for transactions, so it has been renamed and typed accordingly. The key type has also been changed to VertexId, so the origin is removed from it.
• Change None handling in get_tx() from if/raise to assert, as the typing guarantees tx will never be None.

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 #732 (a54f623) into master (405ab6a) will decrease coverage by 0.04%.
The diff coverage is 86.20%.

❗ Current head a54f623 differs from pull request most recent head b602039. Consider uploading reports for the commit b602039 to get more accurate results

@@            Coverage Diff             @@
##           master     #732      +/-   ##
==========================================
- Coverage   84.39%   84.35%   -0.04%     
==========================================
  Files         251      251              
  Lines       21586    21577       -9     
  Branches     2917     2916       -1     
==========================================
- Hits        18218    18202      -16     
- Misses       2737     2742       +5     
- Partials      631      633       +2     

Files Changed	Coverage Δ
hathor/p2p/sync_v2/manager.py	74.09% <86.20%> (-1.49%)	⬇️

... and 4 files with indirect coverage changes