Feature request: support ?version=&version_type= params in update API

[tikitu]

It would be great if the update API supported the same versioning parameters as the index API. The failure semantics is a bit more complex; what I would expect would be:

• version_type=internal: fail if version on getting the doc does not match the version specified; ignore retry_on_conflict and fail immediately on any conflict (a conflict would mean the version after indexing no longer matches)
• version_type=external: fail if version on getting the doc is >= specified version; obey retry_on_conflict while the version in conflict is < the specified version; fail if a retry sees a version >= the version specified.

[martijnvg]

The version and version_type support has been added via #3111, but has only be added to master, so if there is a version conflict then the update requests fails without retrying for both internal and external version types. For the external version type there will only be a conflict when the current version is bigger or equal to the specified version.