Firebase Only: New session after each change to session
### Overview
I'm trying to host the Koa app on Firebase Cloud Functions + Hosting through url rewrites and am having some trouble.

### Firebase issue
Firebase functions + firebase hosting with this causes issues. I noticed that data was getting saved to the provided store (firebase) but it would send a new uuid set-cookie header each time I change any data in the session.

One thing to note is firebase strips all cookies besides '__session' (though i did set this as the 'key' parameter which I assume would solve this)

**Suggested Solution:** 

After looking at the library I am not quite sure what the issue is but I was able to create a temporary solution using firestore as the store. here is an example

`var KoaFirebaseSession = {
  set: async (ctx, key, val) => {
    var ourSessionKey = ctx.cookies.get("__session");
    if(!ourSessionKey) {
      ourSessionKey = uuidv4();
      ctx.cookies.set("__session", ourSessionKey);
    }
    await firestoreAccess.set(ourSessionKey, {
      [key]: val,
      "_expire": Date.now() + COOKIE_CONFIG.maxAge,
      "_maxAge": COOKIE_CONFIG.maxAge
    });
  },
  get: async (ctx, key=null) => {
    var ourSessionKey = ctx.cookies.get("__session");
    if(ourSessionKey) {
      var sessionData = 
        await firestoreAccess.get(ourSessionKey, COOKIE_CONFIG.maxAge);
      if(key && sessionData && sessionData[key]) {
          return sessionData[key];
      } else if(sessionData) {
        return sessionData;
      }
    }
    return {};
  }
}`

 