Support smooth key-rotation, accept multiple public-keys

[rtyley]

We're trying to work towards the ability to refresh the Panda key on a regular basis (eg weekly, or daily) and so we'll need pan-domain-node to handle that!

Work has already been completed in the Scala Panda library to support smooth-key rotation, by accepting multiple public-keys:

• Support accepting multiple public keys pan-domain-authentication#150

...we'll need to duplicate that work in pan-domain-node. This needs two main changes:

• Update the config parsing code in fetchPublicKey() to read the additional public keys defined in alsoAccept.X.publicKey config entries (see sample .settings files)
• Update the cookie verification code in verifyUser() to try each of those additional public keys in turn, rather than just the single active key

See also:

• https://github.com/guardian/data-science-recipes/issues/119