Add support for using aliases to allow hot swapping of indexes.

Allows making the chosen name of an index an aliaswith timestamped versions of the name used as the real name. Will allow populating a new index and then "hot swapping" it by pointing the alias at the new index.

Simplified version of https://github.com/FriendsOfSymfony/FOSElasticaBundle/pull/264 and https://github.com/FriendsOfSymfony/FOSElasticaBundle/pull/376/ to avoid more significant changes. Apologies that this means the original contributors commits are lost.