CDRIVER-4110: Default for causal_consistency depends on snapshot (#838)

* Don't force casualConsistency=false when creating session with snapshot=true

This was originally added in d5f73e18

* Relocate session snapshot prose test

This prose test is defined in the sessions spec, not transactions. It also does not require the server to support transactions.

* Default for causal_consistency depends on snapshot

Use mongoc_optional_t for causal_consistency and snapshot session opts to track whether or not values have been set.

This also removes explicitly setting causal_consistency to true in _mongoc_client_session_new, since mongoc_session_opts_get_causal_consistency now enforces the default value.