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.
with a new endpoint '/sys/audit-hash', which returns the given input
string hashed with the given audit backend's hash function and salt
(currently, always HMAC-SHA256 and a backend-specific salt).
In the process of adding the HTTP handler, this also removes the custom
HTTP handlers for the other audit endpoints, which were simply
forwarding to the logical system backend. This means that the various
audit functions will now redirect correctly from a standby to master.
(Tests all pass.)
Fixes#784
