[ upstream commit 04840b96530031a84bc359c476a59d320617d2db ]

Track which selectors in policy require a specific bpf policy map key to
be present, and keep policy entries in the map as long as any selector
requires it's presence. Without this it is possible for a timed-out
DNS cache entry to clear a policy cache key that is still required by
another selector (FQDN or CIDR).

To implement this, each MapStateEntry is now equipped with a set of
(cached) selectors through which the policy map key/value was
added. 'nil' has the special significance that it is used as the
CachedSelector in cases where the policy map entry is added due to
some administrative or configuration reason. Currently incremental
updates will never remove such entries.

Incremental policy updates now simply collect the requested map
changes. When the endpoint then pulls the changes they are first
applied the desired policy map (MapState), while tallying which
selectors still need the map entries to be present. The actual bpf map
diffs are recorded based on the total count of selectors on each map
entry.

Signed-off-by: Jarno Rajahalme <jarno@covalent.io>