Add listener count to avoid emitter leak warning

[pmacostapdi]

When multiple write nodes are placed in one flow the logs show a warning of the kind:

(node:31) MaxListenersExceededWarning: Possible EventEmitter memory leak detected. 11 stateWrite listeners added to [InfluxDbV2Node]. Use emitter.setMaxListeners() to increase limit

This one-liner increases the maximum number of listeners right before the listener is hooked up and thus no warning is generated.