In this commit, we create a lnrpc.SubServerDriver for signrpc. Note that
this file will only have its init() method executed if the proper build
flag is on. As a result, only if the build flag is set, will the RPC
server be registered, and visible at the packge lnrpc level for the root
server to manipulate.
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.
