Not sure this is required, or not.  Another option is treat it as
protocol error.

Add a counter to rtb_entry

Count the number of times a packet has been retransmitted.
At this stage, it's only purpose is to slowly backoff
retransmit timeouts so as not to cause flooding.
In future it could be used for statistics reporting and
telemetry, especially if a similar counter is kept at
the connection level.

Fix error caused by C warning flags propogating to C++

As some of these are not supported in C++
Additionally, trim DEBUGFLAGS down to focus optimisation level
and debug symbol level only, as --enable-werror will enable
all of the warning flags that will ever be needed

Improve ease of debugging of client/server examples

- Define new CFLAGS in debug mode:
  - enable full embedding of debugging symbols
  - lower optimisation to avoid 'jumping' in gdb
  - enable stricter compiler warnings
- mitigate compiler warnings in client example
- Build client and server executables as binaries to
  allow them to be invoked directly by gdb