Refactor FederationServer

[jillesvangurp]

The current FederationServer is a bit weird in the sense that you create an instance every time you want to validate a single address.

I propose to make this a bit less convoluted as follows:

• FederationAddressResolver that has a resolve method that returns the response and a constructor injected okhttpclient. There should be no need to have separate resolvers for each domain.
• separate the address parsing into a separate static method so that we can test this without needing a mock http server.
• Likewise mocking a remote server to verify that we can deserialize a json response is not something that needs unit testing. At best we need a simple test that verifies that given a valid response, the deserialization works.
• get rid of the static https variable and the static method for creating an okhttp instance; doing this per address is a bad idea. Global variables should not exist in production code.

I think it's safe to do this as obviously not a lot of people are aware this even exists or they would have complained ;-). Happy to do the work if people like this.