isUserSignedIn caches the userSession from shared preferences

[friedger]

The default implementation of BlockstackSession uses SessionStore backed by the default shared preferences.

If two BlockstackSession exists backed by the same preferences and the user is logged out in one session then the second does not reflect that.

[friedger]

A workaround is to use a single instance of SessionStore that is used in both BlockstackSessions.

This needs to be documented or the default SessionStore should not cache the state of the preferences.