Better public key management (build table of valid keys) (#24)

[jw35]

RAVEN076 in master TODO list

[jw35]

The current arrangement (tries to) read Ucam Webauth public keys from the file system each item the module processes an authentication response. Better would be to read all the available keys in at server startup time and then just reference them fro internal storage. The downside would be that a server reload would be needed to load new keys, but that's very rare (too rare, actually...)