To match the previous calling convention for DoJSONHTTPRequest and
reduce confusion when migrating from the old API.

Implement a simpler API for the Backend interface, removing most old
public methods and replacing them with a single Call() method, making
the package look a bit more like an actual rpc package (so, hopefully,
easier to replace in the future).

Creation of sharded/replicated clients was swapped, sigh.

Now the default is for errors to be permanent, unless the error itself
has a Temporary() method that returns true (like the transient network
errors do in the net/ package).

Code 429 is used by the server-side throttling code.