This decouples the RPC server from the internal btcd server to move
closer to being able to split it out into a separate package.
In order to accomplish this, it introduces an rpcserverConfig type and
several new interfaces, named rpcserverPeer, rpcserverConnManager, and
rpcserverBlockManager, which are necessary to break the direct
dependencies on the main server and block manager instances.
It also adds concrete implementations of the new interfaces and uses
them to configure the RPC server.
Ultimately, the RPC server should ideally be decoupled even more such
that all of the types in the configuration struct use interfaces instead
of the concrete types. Doing this would make the RPC server much easier
to internally test since it would allow creating lightweight stubs for
the various pieces.
