This makes it easier to understand the expected lifetime without a
lookup call that uses the single use left on the token.
This also adds a couple of safety checks and for JSON uses int, rather
than int64, for the TTL for the wrapped token.
Add locking around UseToken and Lookup. Have UseToken flag an entry that
needs to be revoked so that it can be done at the appropriate time, but
so that Lookup in the interm doesn't return a value.
The locking is a map of 4096 locks keyed off of the first three
characters of the token ID which should provide good distribution.
Jeff Mitchell