fix: wait for Connect.lookup_or_start_connection to be ready

[edgurgel]

What kind of change does this PR introduce?

This helps with cold starts as multiple websocket connections might happen at the same time.

Scenario 1:

• Connect.lookup_or_start_connection is called once;
• Connect process starts up so syn already registered the process;
• Another Connect.lookup_or_start_connection is called and syn already acknowledges that a Connect for that tenant exists but no conn has been assigned yet to the syn metadata;
  • Current behaviour is to return an error;
  • New behaviour is to subscribe for the Syn event with the updated metadata;
  • If the event arrives we consider that the connection is ready. After 5 seconds we give up.

Scenario 2:

• Connect.lookup_or_start_connection is called from multiple nodes very quickly;
• These RPC calls will most likely land on the same node calling the same DynamicSupervisor on the same partition which means that one of them will proceed creating while the other ones will block and eventually get :already_started;
• This is the happy path because these processes will wait for the first process to finish its init which will already have a conn set.

Additional context

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[coveralls]

coverage: 85.292% (+0.1%) from 85.146%
when pulling a15a1f3 on fix/connect-lookup-cold-start
into 1efdf78 on main.

[kiwicopple]

🎉 This PR is included in version 2.41.6 🎉

The release is available on GitHub release

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