Allow the If-Match header to reference the ETag that resulted from an earlier operation in a batch request

A client may want to group multiple data modification operations of a particular entity in a batch request and use ETags. Currently, the client can use the ETag in the first data modification request but for all subsequent requests in the batch, it would have to use If-Match: *.

We should allow the If-Match header to reference the ETag that resulted in a previous data modification request in a batch (similar to how we use the Content-ID to refer to the URI of an entity).