Mysql rerun dead jobs

[autotaker]

Closes #5

This PR implements missing feature to re-run dead jobs for MySQL backends.

Tests

I added unit tests for this feature. The scenario is the following steps.

Scenario 1

1. Push a job
2. Register a worker with last_seen = (6 minutes ago).
3. Pull the job by the worker.
4. Call heartbeat with ReenqueueOrphaned.
5. Assert that the job status is reset to Pending.

Scenario 2

1. Push a job
2. Register a worker with last_seen = (4 minutes ago).
3. Pull the job by the worker.
4. Call heartbeat with ReenqueueOrphaned.
5. Assert that the job status is not changed.

Other changes

• Extracted method keep_alive_at(worker_id, last_seen) from keep_alive(worker_id) to improve testability
• Modified the condition run_at < NOW() in stream_jobs to run_at <= NOW(), because jobs cannot be pulled immediately after pushed.
• Added Test Suite with MySQL action to ci.yaml to execute unit tests with MySQL.