This pulls the logical request building code into its own function so
that it's accessible from other HTTP handlers, then uses that with some
added logic to the Seal() and StepDown() commands to have meaningful
audit log entries.
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
