Make RoutingTableHistory movable

As part of the changes on SERVER-46199 the new CollectionCache will require that the cached elements (of RoutingTableHistory type) to be movable. To allow this there are three neccessary changes:

• Remove the _sequenceNumber on RoutingTableHistory, that way the RoutingTableHistory merge can be implemented as an inmutable operation, which is necessary for the new lookup pattern to be used for the collection cache
• Remove the const on the RoutingTableHistory class members
• Add a move constructor and a move assign operator