This endpoint causes the node it's hit to step down from active duty.
It's a noop if the node isn't active or not running in HA mode. The node
will wait one second before attempting to reacquire the lock, to give
other nodes a chance to grab it.
Fixes#1093
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
