Add gitaly concurrency queue limit configs

What does this MR do?

Add gitaly concurrency queue limit configs

acb7b76c was reverted because when no value is set for max_queue_wait and max_queue_size, blank values are written which is a malformed toml.

This commit brings back the changes in acb7b76c and fixes this by checking if values exist.

Changelog: added

Checklist

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