vrrp: add stats support and update API

vrrp: add stats support and update API

Add simple counter statistics to VRRP, based on a subset of those
defined in RFC8347.

Add an update API that allows in-place modification of an existing
instance. The method returns a vrrp_index which can be used both for
retrieving statistics and to modify non-key parameters. Also add a
delete method which will take that vrrp_index as parameter.

Type: improvement
Signed-off-by: Emanuele Di Pascale <lele84@gmail.com>
Change-Id: I2cd11467b4dbd9dfdb5aa748783144b4883dba57