IF: Address abstraction leakage in HotStuff library

[arhag]

Specifically:

• fc::unsigned_int usage (which we reserve only for serialization)
• On wire message struct passed throughout the code.

[heifner]

From @ericpassmore : chain_plugin.hpp is included in many places carrying hotstuff with it. When I look for the hs_* structures which also include chain_plugin.hpp I find three files net_plugin producer_plugin chain_plugin. An alternative would be having another header with the hotstuff structures and include the new header in the three files.