Rework RCON statistics gathering

Add a sendRcon method to BaseInstancePlugin and the slave's Instance
class.  This servers two purposes, one is to reduce the repetition when
send RCON commands particularly from plugins, and the other is to
instrument the RCON commands sent with metrics about duration and size.

test/mock.js

@@ -92,6 +92,10 @@ class MockInstance extends libLink.Link {
 			},
 		};
 	}
+
+	async sendRcon(command, expectEmpty, plugin) {
+		return await this.server.sendRcon(command);
+	}
 }
 
 class MockSlave extends libLink.Link {