This commit consolidates several abstractions on the shard level in
ordinary classes not managed by the shard level guice injector.
Several classes have been collapsed into IndexShard and IndexShardGatewayService
was cleaned up to be more lightweight and self-contained. It has also been moved into
the index.shard package and it's operation is renamed from recovery from "gateway" to recovery
from "store" or "shard_store".
Closes#11847
Tanguy Leroux
Igor Motov
Simon Willnauer
Clinton Gormley