refactor(sync-v2): Modify TransactionStreamingClient to process vertices asynchronously

[msbrogli]

Motivation

This PR enhances the on_block_complete() method, which initially processed transactions and added blocks to the DAG in a synchronous manner, thus occupying the event loop for the duration of this task. Due to the synchronous execution, the larger the transaction set, the longer the event loop was blocked, preventing the full node from attending to other critical operations like managing connections and processing API requests.

This PR transforms the method into an asynchronous operation. By doing so, after the addition of each transaction, control is yielded back to the event loop, allowing the full node to multitask effectively. This change is particularly beneficial during periods of heavy transaction processing, ensuring that node responsiveness and efficiency are maintained.

Acceptance Criteria

1. Modify on_block_complete() to yield back to the event loop after calling on_new_tx() for each vertex.
2. Remove the TransactionStreamingClient._buffer because it was not necessary anymore.
3. Even if we receive a TRANSACTIONS-END message, we have to wait until all transactions have been processed before resolving the deferred.

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: 6 lines in your changes are missing coverage. Please review.

Comparison is base (382cc38) 85.42% compared to head (4a7d448) 85.31%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #865      +/-   ##
==========================================
- Coverage   85.42%   85.31%   -0.11%     
==========================================
  Files         282      282              
  Lines       22270    22308      +38     
  Branches     3367     3377      +10     
==========================================
+ Hits        19025    19033       +8     
- Misses       2583     2602      +19     
- Partials      662      673      +11     

Files	Coverage Δ
hathor/p2p/sync_v2/agent.py	78.59% <100.00%> (-0.56%)	⬇️
hathor/p2p/sync_v2/transaction_streaming_client.py	85.50% <88.46%> (-4.69%)	⬇️

... and 6 files with indirect coverage changes

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