udp: add a Map of last use time (ns) -> flow
udp: add a Map of last use time (ns) -> flow

In a future patch we will iterate over the bindings in order to
expire old ones, treating it as a LRU cache.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit