ospf6d: fix neighbor state machine (faster lsdb sync, RFC compliance)

ospf6d: fix neighbor state machine (faster lsdb sync, RFC compliance)

The OSPFv3 code doesn't do the following things right as part of an adjacency
bringup:
 - Transmit DbDesc frames appropriately to ensure faster state transition to
   Loading state
 - Transmit LsReq frames when switching to exchange state and on receipt of
   an LS update in Loading state
 - Requesting LSAs multiple times in LsReq.

It currently uses retransmit timer expiry to send the LsReq and DbDesc frames
which significantly slows down large lsdb syncs.

Signed-off-by: Dinesh G Dutt <ddutt at cumulusnetworks.com>
Reviewed-by: Scott Feldman <sfeldma at cumulusnetworks.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>