TTL indexes can now be created on capped collections, previously this was not allowed.

Description of Linked Ticket

MongoDB doesn't currently allow capped collections to have TTL indexes. The reason dated back to when capped collections used to not replicate explicitly and so it wasn't possible to delete from them and get consistent results. In PM-1858, we want the collections for storing sampled queries (config.sampledQueries and config.sampledQueriesDiff) to be both capped and have a TTL index. These collections currently have TTL indexes but are not capped because of the aforementioned restriction.