Cache user information fetched in the auth middleware

Currently the manager always fetches the user/keypair information from the database in the `auth_middleware`.Let's cache those information using pickle in Redis to reduce the transaction loads.(Since this is just a cache, we can simply fallback to the database query when there are unpickling errors after version updates, database schema changes, etc.)We also need to invalidate the cached entries when the user and keypair data is updated/deleted/purged/deactivated.

Q. Could we utilize the signal subsystem in SQLAlchemy to detect whenever rows in the users/keypairs tables are modified or deleted, without manually adding invalidation triggers all around the codebase?

JIRA Issue: BA-252