use jsonb column for dj handler

[ulferts]

End of the week attempt to store the handler information of delayed job as jsonb instead of yaml as jsonb can be indexed.

I haven't performed extensive tests yet. Especially the serialization of the arguments might prove troublesome.

[oliverguenther]

I did some tests with this PR and performance looks good on the provided job_class index. For SaaS, schema_key would be another thing that might need an index: https://github.com/opf/saas-openproject/pull/103/files#diff-b1f91338b00a9843411d57e72c0197ee98ea419f49008b578702028e5ddf7c50R34

We could do some experiments with the different index types and their storage requirements.

[ulferts]

I tried out various background jobs and they appear to be working fine.

However, given that the release of 12.0 is imminent, I'd rather postpone merging this in just now. Once 12.0 is out, I think it is worth to have it in, though.

[ulferts]

Closing old PRs.