Dynamic server URL in subjects - make moving servers between domains easier

[joepio]

• Create a resource on localhost/myresource
• Host the server on some other place, e.g. example.com
• How can we now access our myresource? We can use the /path endpoint, but we can't go to example.com/myresource.

This behavior happens because all subjects used in Atomic Data (and Atomic Server) are absolute. We could refactor atomic-server to use relative urls, so we could move our server between domains and keep our data. Let's look at some considerations:

• We'll probably need a new Subject datatype (see Change subject / string type arguments #561). When serializing to a client, we should generate the actual URL. We should consider the subdomain here, too. It probably needs Store to properly serialize. This new Subject type will be anywhere.
• When building indexes we should use the compressed format, too. This also means that we should create these compressed representation when building a query.
• External resources pointing to the old domain will no longer work.