isisd: per-instance dynamic hostname cache

isisd: per-instance dynamic hostname cache

Currently, the dynamic hostname cache is global. It is incorrect because
neighbors in different VRFs may have the same system ID and different
hostnames.

This also fixes a memory leak - when the instance is deleted, the cache
must be cleaned up and the cleanup thread must be cancelled.

Fixes #8832.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>