Rewrite the async job system

[matrixbot]

This issue was originally created by @sandhose at matrix-org/matrix-authentication-service#2785.

We're currently using apalis for async jobs, which… is probably not mature enough.
A few issues with it:

• the crates are a bit messy
• they are doing breaking changes all the time. See Upgrade apalis #2784
• the database structure is… weird
• the jobs don't retry correctly
• the "workers" crash when it looses the connection and don't restart
• sharing state in the job is not type safe

Requirements:

• have jobs in the database
• use triggers to NOTIFY workers for new jobs (optional?)
• lock rows with PG's FOR UPDATE SKIP LOCKED
• pass the connection which locked the task row to the handler
• have a way to do cron-like schedules for maintenance tasks

Related:

• Losing the connection to Postgres stops the background jobs matrix-org/matrix-authentication-service#2625
• Ensure background jobs are reliably retrying matrix-org/matrix-authentication-service#1490

### Pull requests
- [ ] #3307
- [ ] https://github.com/element-hq/matrix-authentication-service/pull/3367
- [ ] https://github.com/element-hq/matrix-authentication-service/pull/3455
- [ ] https://github.com/element-hq/matrix-authentication-service/pull/3558
- [ ] https://github.com/element-hq/matrix-authentication-service/pull/3678

### Tasks
- [x] Perform leader election for maintenance tasks
- [x] Shutdown 'lost' workers
- [x] Insert jobs in the database
- [x] Consume jobs from the database
- [x] Retry failed jobs
- [x] Gracefully shutdown workers
- [x] Use a single connection to LISTEN and interact with jobs
- [x] Schedule jobs in the future
- [ ] Re-schedule jobs from lost workers
- [x] Cron-like schedules
- [x] Metrics
- [x] Tracing with span links

[sandhose]

Split the last task in #4491