Mapping validation for system indices

[ppf2]

This is a general enhancement request idea to the connectors framework so that we can avoid future issues like #2264 where a bad schema from index migration ended up causing all sync jobs to fail to start.

1. Each connectors version should have knowledge about the "correct" mappings it expects for its system indices.
2. Implement a mappings validation functionality at startup of the connector service (or at the end of the migration process) so that it can perform an integrity check on the mapping. If the mapping doesn't check out, we can terminate the connector service and throw a fatal exception to report the schema integrity issue so the user can take action.

This will ensure that we fail fast and don't end up with various features breaking in Connectors due to bad mappings in our system indices.