isisd: Deal with SRv6 locator instead of chunk

isisd: Deal with SRv6 locator instead of chunk

Currently, when SRv6 is enabled in IS-IS, IS-IS requests a locator chunk
from Zebra. Zebra assigns a locator chunk to IS-IS, and then IS-IS can
allocate SIDs from the locator chunk.

Recently, the implementation of SRv6 in Zebra has been improved, and a
new API has been introduced for obtaining/releasing the SIDs.

Now, the daemons no longer need to request a chunk.

Instead, the daemons interact with Zebra to obtain information about the
locator and subsequently to allocate/release the SIDs.

This commit extends IS-IS to use the new SRv6 API. In particular, it
removes the chunk throughout the IS-IS code and modifies IS-IS to
request/save/advertise the locator instead of the chunk.

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>