Update API - allow scripted upserts

[markharwood]

In some scenarios it is useful if the update script used to add new content to an existing document is also capable of being called when the document does not already exist (aka Upsert requests).

Now that we have stored scripts they can be non-trivial chunks of code and may have a lot of business logic to manage the state of an item. Object orientation teaches us the value of encapsulation and this enhancement allows the scripts to encapsulate the logic used in the construction of new documents as well as the current logic for modifying existing documents.

The suggested change is that a new scripted_upsert parameter can be passed to upsert requests to indicate if the script should be called to perform inserts. The default value is "false" to reflect the current behaviour. If set to "true" and an insert is being formed the following steps occur:

1. The example upsert document passed by the client (which now could be {} ) is presented in the script context as the initial state of the document held in ctx._source.
2. The ctx.op field in the script context is set to create to indicate that this is an insert as opposed to update operation
3. The script (or stored script) used to perform updates is called with the params values passed from the client (in the same way updates are invoked). It mutates the map of values held in ctx._source appropriately.
4. The script can set ctx.op to none if it wants to ignore the insert, otherwise the document held in ctx._source after the script completes is stored