Add std::unique_ptr<CollatorInterface> to IndexCatalogEntry

• IndexCatalog::_isSpecOk() should fully validate the "collation" field from the index spec.
• IndexCatalogEntry should get a new std::unique_ptr<CollatorInterface> private member.
• IndexCatalogEntry::init() should initialize this private member from the index spec.

Versions of the server prior to 3.4 should not be able to start up if any collection has an index with a non-default collation set. This work will be implemented in SERVER-23761.