Drop old galaxy_session records

[jdavcs]

Ref #13992

Is 1 month a safe cut off for deleting old sessions? If so, main has 173M session records that are more than 2 months old (checked against updated_time; if checked against last_action value, then 151M).

This introduces some minor duplication (see the history pruning script, as well as admin documentation). I'll refactor as needed when adding more such scripts (as per #13992).

A note on prev_session_id:
The galaxy_session.prev_session_id field should be defined as a self-referencing foreign key, but it is not. Instead, it's an integer field which allows null values. Adding a foreign key constraint is trivial; however, the migration process would have to include fixing existing data by setting to null any existing values that do not correspond to an existing galaxy_session record. For databases like main, which contains 173M galaxy_session records, this process requires special handling, and the migration may take a long time. In this case it seems to be not worth the effort.

UPDATE: we do need the previous session id to be available; however, it is not accessed from the code base and, from a practical standpoint, a foreign key constraint is not needed.

To do:

• Check code base for potential problems when session record has been deleted
• Add cascading for: deleting GalaxySessionToHistoryAssociation records, setting to null references on job, event, user_action, galaxy_session (self ref) (model + db migrations)
• Add script to delete session records (also handle references to prev session in galaxy_session table, which is not a foreign key)

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. Upgrade database to enable ondelete behavior
  2. Check galaxy_session records in your current local database
  3. Run script with some cutoff date, verify database that records have been deleted
  4. Run script without setting a date, verify database that records older than one month have been deleted.

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[jdavcs]

test failures unrelated.