Improve Secure Channel shutdown (make Encryptor or Decryptor remove itself from the Registry)

[SanjoDeundiak]

Current behavior

Secure Channel consists of a Encryptor Worker and a Handshake/Decryptor Worker. Currently, the proper way to stop a secure channel is implemented here, so both of the Workers are shutdown in that function. Also, the corresponding entry is removed from the Registry. However, that introduces room for inconsistency since a library user may not know about this function or not have access to it (if they don't have access to the SecureChannels instance). In this case, it's very tempting to use ctx.stop_worker() API and shut down only one of the Workers (probably, Handshake/Decryptor Worker, because we primarily use the Encryptor Address after establishing a Secure Channel) or forget to remove the Registry entry.

Desired behavior

The possibility for that mistake could be avoided if calling ctx.stop_worker() with either Encryptor or Decryptor Address would shut down the Secure Channel properly. To achieve that 2 things are needed:

1. Make Encryptor and Decryptor shut down each other in their shutdown functions here and here. Which is Improve Secure Channel shutdown (make Encryptor and Decryptor shut down each other) #5322
2. Make one of these Workers remove its entry from the Registry in its shutdown function. Which is this issue.

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