Allow session to be cached to disk

[tristanpenman]

Raising this as a draft, for some early feedback. This is just my first pass at doing this, and I feel that some of the error cases are not yet handled well enough.

A quick summary of the changes so far:

• Allows session cookies and headers to be cached to disk
• Attempts another login if call to update_token fails
• Session cache also stores hash for username, password, and TOTP secret, so that if any of the original unhashed values change, we know that the cached session cannot be used

Keen to hear what you think!