An async write operation completion problem

[rwaldron]

The problem begins here: rwaldron/johnny-five#1306 (important to read through to my findings at the end).

Summary points:

• serialport writing is non-blocking
• firmata.js dependent programs may want to do cleanup tasks before exiting.
• firmata.js needs a simple way to signal that writes are:
  • pending
  • complete
• incrementing and decrementing a counter is simple way to achieve such signaling.