slurmdbd: Implement Association API

[tazend]

Classes

• [x] Association class providing access to all possible members of slurmdb_assoc_rec_t
• [x] Associations class, acting as a custom collection (dict) that holds all the Association instances.
• [x] AssociationFilter class acting as a wrapper around slurmdb_assoc_cond_t to search for associations matching specific conditions (server-side SQL filter).

General Functionality for the Associations class

• [x] modify
• [x] load
• [ ] delete
• [ ] create

Other

• [ ] Documentation
• [ ] Write tests
• [ ] Write examples