Use hashed token for invalid PKI token cache key

Invalid PKI tokens are cached in memcache using the entire token as
key. This triggers the familiar memcache key length error since a
PKI token is much longer than 250 characters.

This change hashes the token before using it as a key, and also changes
instances of "token" to "token_id" where appropriate so it's clear
that we're passing around a hashed token rather than the token data
itself.

Change-Id: I765b209578d60266da706094f61d8d9b15cfb6de
Closes-bug: 1206347
Merge "Use hashed token for invalid PKI token cache key"