feat(queue): add indexing worker implementation

[msm-cert]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running mquery, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

Indexing files requires fiddling with scripts directly on ursadb container/server

What is the new behaviour?

Indexing queues are getting operational. The next step is testing them internally and migrating to them.

Test plan

1. use POST /api/queue/{urasdb_id} to add a file
2. use POST /api/queue/{urasdb_id}/index to start indexing
3. check that indexing job was started

[msm-cert]

Related to #410