completion: Add a callback upon completion
completion: Add a callback upon completion

This callback will be called when the completion socket is closed. It
allows an interested party to immediately apply some behaviour at the
time that the completion occurs, rather than waiting on the completion
in the outer code. It will be used in an upcoming commit to reflect the
policy revision from proxy acknowledgements up to the endpoint.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@covalent.io>

cilium-team/cilium,eloycoto/cilium,eloycoto/cilium,cilium/cilium,scanf/cilium,cilium-team/cilium,tklauser/cilium,tgraf/cilium,scanf/cilium,michi-covalent/cilium,tklauser/cilium,eloycoto/cilium,cilium/cilium,michi-covalent/cilium,cilium/cilium,scanf/cilium,tgraf/cilium,michi-covalent/cilium,scanf/cilium,tgraf/cilium,scanf/cilium,michi-covalent/cilium,tklauser/cilium,eloycoto/cilium,michi-covalent/cilium,tklauser/cilium,cilium/cilium,scanf/cilium,tgraf/cilium,tgraf/cilium,eloycoto/cilium,scanf/cilium,cilium/cilium,eloycoto/cilium,eloycoto/cilium,tklauser/cilium,tgraf/cilium