Add future development notes to DEVELOP.md

    * System tests link/run against the full kinetic-c static library.

    * A generic test fixure is provided and linked into each system test from: `test/support/system_test_fixture.h/c`

        * See details above for runtime configuration of the system test kinetic devices for running remote simulators or kinetic device hardware.

Future development notes

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* epoll(2) could be used internally, in place of poll(2) and multiple

  listener threads. This only matters in a case where there is a large

  amount of idle listener connections. When there is a small number of

  file descriptors, it will add overhead, and epoll is only available

  on Linux.

* The listener can potentially leak memory on shutdown, in the case

  where responses have been partially received. This has been a low priority. 

* There is room for tuning the total number of messages-in-flight

  in the listener (controlled by `MAX_PENDING_MESSAGES`), how the

  backpressure is calculated (in `ListenerTask_GetBackpressure`), and the

  bit shift applied to the backpressure unit (the third argument to

  `bus_backpressure_delay`, e.g. `LISTENER_BACKPRESSURE_SHIFT`). These

  derive the feedback that pushes against actions that overload the

  system. The current setup has worked well with system/integration

  tests and a stress test program that attempts to overload the message

  bus over a loopback connection, but other workloads may have different

  performance trade-offs.