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
7eb8c7d765