Switch custom component serialization in ClusterState from PROTO to NamedWriteable

[imotov]

We currently have two different ways to serialize/deserialize objects with variable types. The older method, which is used in cluster state serialization is using registered PROTO objects to select the proper de-serialization reader. In all other places we switched to using NamedWriteables for the same purpose.

I would like to convert custom objects in ClusterState, MetaData and IndexMetaData to NamedWritables and see if I can completely remove PROTO-based serialization to make cluster state serialization/deserialization more in line with the rest of the system. It's a significant change internally and will affect plugins that are registering custom metadata objects, but it looks like wire-protocol would remain the same and therefore it should be possible to make this change compatible with rolling restart.

[nik9000]

I didn't take this on when I was converting stuff to NamedWriteable a while back just because I had enough stuff to do converting what I did convert. I think this makes sense if you can keep the wire protocol the same.

[s1monw]

lets do this it's a good cleanup