Reduce memory usage for XML-RPC requests

[josephscott]

Demitrious Kelly did some excellent work in reducing the memory usage for processing large XML-RPC requests on WordPress.com. I'm including a patch of his work on wp-includes/class-IXR.php that does the following:

• Pass references instead of copies of the full XML-RPC request
• Limit the XML declaration stripping to the first 100 characters of the data
• Parse the XML in chunks so that the whole thing doesn't get slurped into memory

(In [12263]) Reduce memory usage for XML-RPC requests. Props Demitrious Kelly. fixes #10698