Collator-side of collator protocol (#351)

* skeleton of collators object

* awaiting and handling collations. rename `collators` to CollationPool

* add some tests

* add tests

* implement Collators trait for ConsensusNetwork

* plug collators into main polkadot-network

* ignore collator role message

* add a couple more tests

* garbage collection for collations

* extract session-key tracking from consensus

* add local_collations.rs

* finish polish of local_collations

* integrate local_collations into network layer

* introduce API for adding local collations

* mostly finish collator implementation pending service fix

* Specialized network()

* push collations to the network

* grumbles

* substrate-service has custom configuration

* initialize network in collator mode as necessary