db: prevent duplicate jobs

Created by: asdine

When enqueueing a job we make sure to replace queued job with the new one if they are for the same couple project / external service id.

Note: We will probably need an index, I will add one in another PR. Index added here https://github.com/sourcegraph/sourcegraph/pull/36986

Related to #36450 (closed)

Test plan

Improved a unit test