DATAMONGO-963 - CompoundIndex should treat expireAfterSeconds correctly.

We added an additional check on the fields used as key, so that TTL is ignored for `CompoundIndex` with more than one field.

Prior to this change potentially invalid index structures would have been created for eg. `@CompoundIndex(def = "{'foo': 1, 'bar': 1}", expireAfterSeconds=10)` leading to constantly recurring error _key for ttl index can only have 1 field_

This fix is related to: [SERVER-10075](https://jira.mongodb.org/browse/SERVER-10075).