Obsolete parameters and APIs that are removed in Elasticsearch 5.0

[russcam]

_timestamp
_ttl

...etc

[gmarz]

I've linked a few issues here for the major stuff we need to mark obsolete, but we can also use the breaking + v5.0.0 label.

[russcam]

Started adding [Obsolete] attributes in branch https://github.com/elastic/elasticsearch-net/tree/fix/deprecated-mappings