The Identity Provider's Service Provider cache had two issues:

1. It checked for identity based on sequence numbers, but didn't
   include the `seq_no_primary_term` parameter on searches, which
   means the sequence would always by `-2`
2. It didn't track whether the index was deleted, which means it
   could be caching values from an old version of the index

This commit fixes both of these issues.

In practice neither issue was a problem because there are no
deployments that use index-based service providers, however the 2nd
issue did cause some challenges for testing.