fix(sync-v2): Fix issues caused by concurrent syncing peers

[msbrogli]

Motivation

Sync-v2 had some issues when syncing with multiple peers. It was causing some tests with three or more peers to be flaky. Notice that peers could still sync but it was taking longer because of the reconnections.

Acceptance Criteria

1. Fix on_block_complete() to handle vertices that already exists. It might happen because other peer had added these vertices before the current peer.
2. Fix BlockchainStreamingServer.send_next() which was sending one block more than the limit.
3. Fix TransactionStreamingClient.process_tx() which was not correctly handling dependencies that already exists in the storage. This condition might happen because of a resume or because other peer had added these dependencies before the current peer.
4. Always add vertex to _existing_deps if it already exists. This is necessary because the streaming server does not know we already have it so it will always send the whole sub-DAG of transactions.
5. Improve logging.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (090bd71) 85.42% compared to head (59b291c) 85.37%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #872      +/-   ##
==========================================
- Coverage   85.42%   85.37%   -0.06%     
==========================================
  Files         281      281              
  Lines       22353    22351       -2     
  Branches     3388     3387       -1     
==========================================
- Hits        19096    19083      -13     
- Misses       2582     2590       +8     
- Partials      675      678       +3     

Files	Coverage Δ
hathor/p2p/sync_v2/agent.py	80.15% <100.00%> (-0.41%)	⬇️
hathor/p2p/sync_v2/streamers.py	86.59% <100.00%> (+0.30%)	⬆️
hathor/p2p/sync_v2/transaction_streaming_client.py	86.61% <88.88%> (-0.80%)	⬇️

... and 6 files with indirect coverage changes

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