Indexed CAS -- FINAL

Description

This is the final MR for indexed CAS.

Old discussion here: !231 (closed)

Collapsed discussions here: !241 (merged) !242 (merged) !243 (merged)

Changes proposed in this merge request:

• Add Index models
• Add an Index interface
• Add a SQL implementation for the index and many tests
• Add configuration to the YAML parser for the SQL index

This merge request, when merged, will address issue/bug:

#181 (closed)