Rewrite network protocol/service to use channels (#1340)

* rewrite network protocol/service to use channels

* remove use of unwrap

* re-introduce with_spec

* remove unnecessary mut

* remove unused param

* improve with_spec, add with_gossip

* rename job to task

* style: re-add comma

* remove extra string allocs

* rename use of channel

* turn TODO into FIXME

* remove mut in match

* remove Self in new

* pass headers by value to network service

* remove network sender from service

* remove TODO

* better expect

* rationalize use of network sender in ondemand