codeintel: Remove job handle abstraction
Created by: efritz
This PR removes the codeintel jobHandle class, which is more verbose than necessary to accomplish what it does (a simple wrapper around an identifier).
Summary of changes:
- db.Savepoint now generates an ID (as job handle did)
- db.RollbackToSavepoint now keeps track of valid savepoint identifiers in the current txn
- db.Dequeue now returns a transactional DB instead of a job handle
- db.Dequeue refactored so that adding additional work type s(indexes) will be easier in the near-future
- Updated worker to use new signatures