Sentry Solr to support multi-attribute document level security

This is an improvement request to cover enhanced document level security for the Solr document level controls, specifically to cover:

1. Security controls against multiple fields
2. Filters based on user attributes as well as just Sentry roles
3. Different security predicates (AND, LessThan, GreaterThan - in addition to the currently supported OR)
4. Pluggable user attribute source ahead of Sentry enhancements.
5. Sample LDAP user attribute source

The ambition is this will be a precursor to full complex predicate support being served by Sentry ABAC roadmap.