Refactor TLS configuration
Refactor TLS configuration

Allow the administrator to specify a Certification Authority (CA) to be
used to verify the peer in TLS communication.

Remove parameters from `agent` and `analyzer`. Instead use `tls` namespace
to define:

```
tls:
    client_cert: /path/to/pem
    client_key: /path/to/key
    ca_cert: /path/to/ca
    server_cert: /path/to/pem
    server_key: /path/to/key
```

This makes the code and configuration easiser to understand and maintain.

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