Only read TLS CA once

[dmitrii-ubskii]

What is the goal of this PR?

We modify the cluster server client so that it only reads the TLS root CA once (if provided), and reuses the SSL credentials when prompted for a new channel.

What are the changes implemented in this PR?

Drive-by: don't crash trying to access auth token before the stub is created.

Closes #280. Closes #273.

[typedb-bot]

PR Review Checklist

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Trivial Change

• [x] This change is trivial and does not require a code or architecture review.

Code

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