Remove support for the TTL and Timestamp metafields

[jbaiera]

According to the breaking changes in 6.0:

timestamp and ttl are not accepted anymore as parameters of index/update requests.

We should be deprecating mentions to these in 5.x and removing them in 6.0

[jbaiera]

Moving forward with deprecating the tags in 5.5 and adding verification logic in 6.0 to disallow their usage against ES clusters running 6.x and above.