net: Rename send_queue to incoming_queue

Each networking client has a queue for packets that could not yet be
delivered to that client. Calling this queue "send_queue" is highly
confusing as it has nothing to to with packets send from this client but
to it. Avoid this confusing by renaming it to "incoming_queue".

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>