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