Tests for new network code (#897)

* move protocol.rs into subfolder

* add trait for mocking network behavior

* add a mock version of network ops

* remove some redundant parameters from service messages

* ensure fetching erasure chunks automatically cancels

* introduce dummy ProvideRuntimeApi

* abstract over gossip somewhat

* add mock gossip handler

* skeleton test

* remove dependence of shared table on router

* remove worker dependence on its own sender

* test shutdown

* add tests

* test that gossip streams are cleaned up correctly

* refactor worker out into its own struct and reduce bound on executor

* remove reliance of tests on global thread pool