XML-RPC Push: Prevent duplicate meta on updates.

Because of how XML-RPC works, we can only update existing meta fiels by
specifying IDs we want to override and their new values.

Currently, we don't do this, so updates to pushed posts result in
duplicated meta.

Let's avoid this by first deleting existing meta using IDs, then
re-inserting all the updated meta fields. This way, we can handle adds,
updates, and deletes.

Fixes #35