* 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