Add lock to Pubsub.execute_command to ensure only one connection is created

[dlunch]

Pull Request check-list

• [v] Do tests and lints pass with this change?
• [v] Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• [v] Is the new or changed code fully tested?
• [] Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• Is there an example added to the examples folder (if applicable)?
• Was the change added to CHANGES file?

NOTE: these things are not required to open a PR and can be done
afterwards / while the PR is open.

Description of change

If multiple threads sharing single pubsub instance calls pubsub command simultaneously, connection leak is observed from pubsub's execute_command function.

This PR fixes the issue by adding the lock on connection fetch.