Several time related improvements:

Several time related improvements:
- set initial delay to 0 to speed up client start
- added 'initial-delay' parameter to possibly revert to old behavior
- better handling of very short (1 or 2s) leases
- client lease records are recorded at most once every 15 seconds
- ICMP ping-check is now timed more precisely
- Servers that don't offer lease-time are now black-listed
[ISC-Bugs #19660]