Implement asyncio timer and deltimer commands

[jghibiki]

Re-implement the existing C++ timer and deltimer commands

Design Details:

Timer command description:

• schedules a timer to execute in the future
• syntax: timer -r [time in ms] [command] e.g. timer -r 500 echo hello
• the -r flag is optional, and causes the timer to repeat/reschedule
• returns the id of the timer.

deltimer command description:

• cancels an existing timer by id
• syntax timer [timer id]

Design Notes:

• to wait with asyncio, call await asyncio.sleep(2) to sleep 2 seconds. This is preferable to timer.sleep() as the timer method halts the thread.
• consider asyncio.wait_for for waiting on the event queue with a timeout.
• you should be able to use the TextInputEvent type to propagate the command text, and let the command module do the parsing and dispatching for you.

Done when:

• A user can schedule a command and have it called after a given time period
• A user can schedule a command to repeat every so often
• A user can delete a timer by id.