feat(emitter): flush remaining effects queue on state update

Previously, if a state update arrived while an effects queue was in the middle of being processed,
the active effects queue was simply discarded and the new one processed. This was potentially buggy
in that `onEnd` callbacks might not be called even if the `onStart` was (and similar clean-up bugs
if relying on a separate effect or `effects:end` to run clean-up, stop animations etc.). Now, any
outstanding effects are flushed when a new state update arrives, ensuring those callbacks will all
immediately be called before the new queue is started.

BREAKING CHANGE: This may be a breaking change if you were relying on the previous queue being
discarded. Given that would be highly timing dependent between when moves where executed, that seems
unlikely to be the case, but flagging this as potentially breaking nonetheless.

Closes #57

src/emitter/emitter.ts

@@ -150,6 +150,7 @@ class EffectsEmitterImpl<S extends ClientState> implements EffectsEmitter<S> {
    * Update the queue state when a new state update is received from boardgame.io.
    */
   public onUpdate(state: null | S): void {
+    this.flush();
     const prevState = this.latestState;
     this.latestState = state;
     if (