REST API: Avoid using 'parent' as path argument name for autosaves.

When 'parent' is set as the path argument name, it gets passed down through to the `create_item()` method and can erroneously reset the 'parent' value on the post itself. Instead, we rename the argument to 'id' and replicate the revision controller's `get_items_permissions_check()` to instead reference 'id'.

Also ensures revision query params (of which there are many) aren't exposed as the query params for autosaves (of which there are two).

Props TimothyBlynJacobs.
Merges [43897] to trunk.
See #43316.
Built from https://develop.svn.wordpress.org/trunk@44250


git-svn-id: https://core.svn.wordpress.org/trunk@44080 1a063a9b-81f0-0310-95a4-ce76da25c4cd