Template abstract_clock on Clock:
The abstract_clock is now templated on a type meeting the requirements of
the Clock concept. It inherits the nested types of the Clock on which it
is based. This resolves a problem with the original design which broke the
type-safety of time_point from different abstract clocks.
Remove unused chrono::time_point stream conversions
Move the call to cancel_timer to the right place
Fixes to beast::asio::streambuf:
* Fix to_string conversion
* Fix assert on debug invariant checks
* Fix the treatment of the output position when the entire output is committed.
* Add unit test
Use injections from OverlayImpl in PeerImp
Use cluster state in Slot instead of PeerImp
Refactor protocol message parsing:

This replaces the stateful class parser with a stateless free function.
The protocol buffer message is parsed using a ZeroCopyInputStream.

* Invoke method is now a free function.
* Protocol handler doesn't need to derive from an abstract interface
* Only up to one message is processed at a time by the invoker.
* Remove error_code return from the handler's message processing functions.
* Add ZeroCopyInputStream implementation that wraps a BufferSequence.
* Free function parses up to one protocol message and calls the handler.
* Message type and size can be calculated from an iterator
  range or a buffer sequence.