Add job queue RPC functions

jobqueue_update: Uploads a job queue file's content to a node. The
most common operation is to upload something that we already have
in a string. Unlike in the upload_file function, the file is not
read again when distributing changes, but content has to be passed
as a string.

jobqueue_purge: Removes all queue related files from a node.

Reviewed-by: iustinp