feat(sync-v2): Fix issue when a reorg occurs during a streaming of transactions

[msbrogli]

Motivation

An unhandled exception is raised by current_block.get_next_block_best_chain() when a reorg occurs during a streaming of transactions. This unhandled exception causes the send_next() to stop being called because it is not scheduled to run anymore, so the other peer stays hanging waiting for more transactions.

Acceptance Criteria

1. Check if block is still in the best blockchain before calling get_next_block_best_chain(). If not, stop the streaming.
2. Add a safe_send_next() that will either schedule the next call or stop the streaming.
3. Add StreamEnd.INTERNAL_ERROR.

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

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (db38db4) 85.33% compared to head (df21071) 85.43%.

❗ Current head df21071 differs from pull request most recent head 9fff604. Consider uploading reports for the commit 9fff604 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #855      +/-   ##
==========================================
+ Coverage   85.33%   85.43%   +0.09%     
==========================================
  Files         282      282              
  Lines       22247    22253       +6     
  Branches     3363     3363              
==========================================
+ Hits        18984    19011      +27     
+ Misses       2595     2576      -19     
+ Partials      668      666       -2     

Files	Coverage Δ
hathor/p2p/sync_v2/streamers.py	87.30% <37.50%> (-3.56%)	⬇️

... and 7 files with indirect coverage changes

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