Fix replication setup

The change notifications we get back from the plugins API mangles
the raw CouchDB users doc into a Hoodie users doc. Most notably.
the _id field is turned into id and the CouchDB user prefix
org.couchdb.user: and the Hoodie user/ type prefix are stripped.

E.g. a user org.couchdb.user:a will have the field id:"a".

The global shares worker code here constructs URLs to the users
database expecting _id to be present. This change makes the code
create the right URLs.