During CNIServer reconciliation, we perform host-local IPAM garbage
collection (GC) by comparing the set of IPs allocated to local Pods and
the set of IPs currently reserved by the plugin. We release any IP
reserved by the plugin that is not in-use by a local Pod. The purpose is
to avoid leaking IP addresses when there is a bug in the container
runtime, which has happened in the past.

Two key design choices that were made:
* We do not invoke CNI DEL to release IPs, instead we access the
  host-local data which is persisted on the Node, and modify it as
  needed.
* We do not rely on the interface store (as persisted to OVSDB) to
  determine the set of IPs that may have been leaked. In case of an
  Antrea bug, it could be possible (although unlikely) for an IP to
  still be allocated by host-local but be missing from the interface
  store. Intead, we list all allocated IPs from the host-local data (an
  allocated IP corresponds to one disk file).

This approach is essentially the same as our existing script:
https://github.com/antrea-io/antrea/blob/main/hack/gc-host-local.sh

Fixes antrea-io#4326

Signed-off-by: Antonin Bas <abas@vmware.com>

Signed-off-by: Antonin Bas <abas@vmware.com>