Add new (Tls)RpcSend funcs to send a Cmd.

The existing RpcCommand and TlsRpcCommand were build with
CreateMessage(WithID) in mind since they return an already marshalled
command.  The approach was changed to use concrete commands created via
the New<command>Cmd family of functions.  However, this means the caller
had to manually marshal the command before calling (Tls)RpcCommand which
was not very user friendly.

This commit adds two new functions named RpcSend and TlsRpcSend which
directly accepts a btcjson.Cmd and handles the marshalling itself.

ok @jcvernaleo