lnrpc: add new sub RPC server registration system
lnrpc: add new sub RPC server registration system

In this commit, we add the scafolding for the future sub-server RPC
system. The idea is that each sub server will implement this particular
interface. From there on, a "root" RPC server is able to query this
registry, and dynamically create each  sub-sever instance without
knowing the details of each sub-server.

In the init() method of the pacakge of a sub-server, the sub-server is
to call: RegisterSubServer to claim its namespace. Afterwards, the root
RPC server can use the RegisteredSubServers() method to obtain a slice
of ALL regsitered sub-servers. Once this list is obtained, it can use
the New() method of the SubServerDriver struct to create a new
sub-server instance.

Each sub-server needs to be able to locate it's primary config using the
SubServerConfigDispatcher interface. This can be a map of maps, or a
regular config structr. The main requirement is that the sub-server be
able to find a config under the same name that it registered with. This
string of abstractions will allow the main RPC server to find, create,
and run each sub-server without knowing the details of its configuration
or its role.

LightningNetwork/lnd,LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd