Attempt to merge two fixes and three new features (custom async token storage, userReady flag, loginWithCustomHandler)

[bratelefant]

Hi all,

for my app I came up with a few tweaks for this package and I try to realign my changes with the master branch of this package again. I tested the features in my staging env, no issues so far. I'll open this as a draft so we can discuss if we want to merge this into master.

Summary

• fix: memorize initial value of autoReconnect option
• fix: race condition that might break a resuming session
• add option.AsyncTokenStorage (eg. for using Expos SecureStorage)
• add userReady() method to determine, if the initial user object finished loading
• add loginWithCustomHandler to be able to call custom Meteor loginHandlers

Linked issue(s)

Fixes #164

Involved parts of the project

User.js and Meteor.js mainly

Added tests?

no additional tests so far