ospf6d: Handle Premature Aging of LSAs

ospf6d: Handle Premature Aging of LSAs

RFC 2328 (14.1) Premature aging of LSAs from
routing domain :
When ospf6d is going away (router going down),
send MAXAGEd self originated LSAs to all
neighbors in routing domain to trigger
Premature aging to remove from resepective LSDBs.

Neighbor Router Reboot:
Upon receiving Self-originate MAXAGEd LSA, simply
discard, Current copy could be non maxaged latest.

For neighbor advertised LSA's (current copy in LSDB)
is set to MAXAGE but received new LSA with Non-MAXAGE
(with current age), discard the current MAXAGE LSA,
Send latest copy of LSA to neighbors and update the
LSDB with new LSA.

When a neighbor transition to FULL, trigger AS-External
LSAs update from external LSDB to new neighbor.

Testing:
R1 ---- DUT --- R5
| \
R2 R3
|
R4

Area 1: R5 and DUT
Area 0: DUT, R1, R2, R3
Area 2: R2 R4

Add IPv6 static routes at R5
Redistribute kernel routes at R5,
Validate routes at R4, redistributed via backbone
to area 2.
Stop n start frr.service at R5 and validated
MAXAGE LSAs then recent age LSAs in Database at DUT-R4.
Validated external routes installed DUT to R4.

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>