Improve root message handling and interface

[jakekidd]

• Added immutable flag IS_HUB set on instantiation.
• Renamed sendMessage / receiveMessage to sendRoot / receiveRoot to make interface less confusing (we already have process methods)
• Renamed related events as well.
• sendRoot takes no arguments and just sends the current outboundRoot
• Virtual method _receiveRoot, which should be implemented by the specific <Domain>Connector, now returns the new updated root (bytes32). receiveRoot calls internal _receiveRoot, sets the aggregateRoot to the returned value, and if IS_HUB is true, will also update the RootManager (by calling setOutboundRoot(mirrorDomain, aggregateRoot))
• Added method propagateRoot which can only be called by RootManager on the hub chain (i.e. if IS_HUB is true)