fix(sync-v2): adjust init assertion

[jansegre]

Motivation

During the QA of sync-v2 some errors were caught during a sync from scratch (see here). In particular this one:

2023-11-12 05:10:57 [warning  ] [hathor.p2p.protocol] send error                     msg=internal error peer_id=7181d23 remote=127.0.0.1:9001
2023-11-12 05:10:57 [error    ] [hathor.p2p.sync_v2.agent] unhandled exception            peer=7181d23
Traceback (most recent call last):
  File "/Users/jan/Projects/hathor-core/hathor/p2p/sync_v2/agent.py", line 296, in run_sync
    yield self._run_sync()
  File "/Users/jan/Library/Caches/pypoetry/virtualenvs/hathor-JIQwCn9w-py3.11/lib/python3.11/site-packages/twisted/internet/defer.py", line 1693, in _inlineCallbacks
    result = context.run(
             ^^^^^^^^^^^^
  File "/Users/jan/Library/Caches/pypoetry/virtualenvs/hathor-JIQwCn9w-py3.11/lib/python3.11/site-packages/twisted/python/failure.py", line 518, in throwExceptionIntoGenerator
    return g.throw(self.type, self.value, self.tb)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jan/Projects/hathor-core/hathor/p2p/sync_v2/agent.py", line 311, in _run_sync
    is_block_synced = yield self.run_sync_blocks()
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jan/Library/Caches/pypoetry/virtualenvs/hathor-JIQwCn9w-py3.11/lib/python3.11/site-packages/twisted/internet/defer.py", line 1697, in _inlineCallbacks
    result = context.run(gen.send, result)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jan/Projects/hathor-core/hathor/p2p/sync_v2/agent.py", line 397, in run_sync_blocks
    reason = yield self.start_transactions_streaming(partial_blocks)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jan/Projects/hathor-core/hathor/p2p/sync_v2/agent.py", line 834, in start_transactions_streaming
    self._tx_streaming_client = TransactionStreamingClient(self,
                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/jan/Projects/hathor-core/hathor/p2p/sync_v2/transaction_streaming_client.py", line 87, in __init__
    assert self._waiting_for
AssertionError
2023-11-12 05:10:57 [info     ] [hathor.p2p.protocol] disconnected                   peer_id=7181d23 reason=Connection was closed cleanly. remote=127.0.0.1:9001

This indicates that the assertion inside TransactionStreamingClient.__init__ is not correct.

Acceptance Criteria

• Adjust TransactionStreamingClient.__init__ assertion to account for _existing_deps and not only _waiting_for.

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

All modified and coverable lines are covered by tests ✅

Comparison is base (29a622f) 85.29% compared to head (e09351c) 85.26%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #875      +/-   ##
==========================================
- Coverage   85.29%   85.26%   -0.03%     
==========================================
  Files         281      281              
  Lines       22364    22363       -1     
  Branches     3388     3388              
==========================================
- Hits        19075    19068       -7     
+ Misses       2610     2608       -2     
- Partials      679      687       +8     

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