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

substrate/polkadot/cli/src/cli.yml

@@ -29,10 +29,6 @@ args:
       value_name: KEY
       help: Specify node secret key (64-character hex string)
       takes_value: true
-  - collator:
-      long: collator
-      help: Enable collator mode
-      takes_value: false
   - validator:
       long: validator
       help: Enable validator mode