inspect: add inspect mode for debugging crashed tendermint node (#6785)

EDIT: Updated, see [comment below]( https://github.com/tendermint/tendermint/pull/6785#issuecomment-897793175)

This change adds a sketch of the `Debug` mode. 

This change adds a `Debug` struct to the node package. This `Debug` struct is intended to be created and started by a command in the `cmd` directory. The `Debug` struct runs the RPC server on the data directories: both the state store and the block store.

This change required a good deal of refactoring. Namely, a new `rpc.go` file was added to the `node` package. This file encapsulates functions for starting RPC servers used by nodes. A potential additional change is to further factor this code into shared code _in_ the `rpc` package. 

Minor API tweaks were also made that seemed appropriate such as the mechanism for fetching routes from the `rpc/core` package.

Additional work is required to register the `Debug` service as a command in the `cmd` directory but I am looking for feedback on if this direction seems appropriate before diving much further.

closes: #5908

light/proxy/proxy.go

@@ -113,7 +113,7 @@ func (p *Proxy) listen() (net.Listener, *http.ServeMux, error) {
 	}
 
 	// 4) Start listening for new connections.
-	listener, err := rpcserver.Listen(p.Addr, p.Config)
+	listener, err := rpcserver.Listen(p.Addr, p.Config.MaxOpenConnections)
 	if err != nil {
 		return nil, mux, err
 	}