Add primary support for multi-location providers

In order to support failover, EJB and other frameworks need a provider-agnostic way to know that the provider was configured with multiple URLs and a way to access those URLs from the base provider interface.

As a part of this, the single and multi-URL providers for the Remoting implementation should be flattened and support for load balancing among locations should be pushed up to the context implementation (in this case, transactions, remote JNDI, EJB, and the HTTP equivalents of these).