endpointCnt is a refcounter used to track how many endpoints use a
network, and how many networks references a config-only network. It's
stored separately from the network.
This is only used to determine if a network can be removed.
This commit removes the `endpointCnt` struct and all its references. The
refcounter is replaced by two lookups in the newly introduced `networks`
and `endpoints` caches added to the `Controller`.
Signed-off-by: Albin Kerouanton <albinker@gmail.com>
