CIDEVSTC-201,202,203: Pyon enhancements for Postgres

This PR delivers these changes:
- Refactored container datastore layer to support different datastore backends (CIDEVSTC-202)
- Refactored couchdb datastore to comply with new datastore framework
- Added PostgreSQL datastore backend (CIDEVSTC-201)
- Provide Postgres search capabilities and geospatial column handling (CIDEVSTC-203)
- Added couchbase datastore backend (experimental)
- Added postgres and neo4j prototypes