Celery tasks should get a task-scoped session

We scope sqlalchemy sessions based on the request id, or else by thread (the sqlalchemy default) in https://github.com/galaxyproject/galaxy/blob/cdcf3c147972625f3db6c02c9ae23dc42659addc/lib/galaxy/model/base.py#L38

We should probably ensure that celery tasks receive a fresh session that is discarded after the task has completed.
xref https://github.com/galaxyproject/galaxy/pull/12963#discussion_r754421690

Start with lib/galaxy/model/base.py at line 38 to understand the existing request- and thread-scoped SQLAlchemy sessions, then review the referenced discussion in pull request 12963. Done means Celery tasks receive a fresh session and that session is discarded when each task completes.

Written by the indexing model from the issue text.