Multiple items to clean up IPv6 address processing.

Multiple items to clean up IPv6 address processing.
When processing an IA that we've seen check to see if the
addresses are usable (not in use by somebody else) before
handing it out.
When reading in leases from the file discard expired addresses.
When picking an address for a client include the IA ID in
addition to the client ID to generally pick different addresses
for different IAs.
[ISC-Bugs #23138] [ISC-Bugs #27945] [ISC-Bugs #25586]
[ISC-Bugs #27684]