rename _shard -> _index and also rename classes and variables

closes #4584
Check if node is still present when collecting attribute shard routings

The node we need to lookup for attribute colelction might not be part
of the `DiscoveryNodes` anymore due to node failure or shutdown. This
commit adds a check and removes the shard from the iteration.

Closes #4589
Use `recycler` prefix for CacheRecycler settings

Currently the cache recycler uses top level settings for it's
configuration. We should prefix them with a `recycler` to make sure
we don't have any confilicts.
Use component settings from the base class