Kill subprocesses using psutils

[DonJayamanne]

When debug adapter ends, ensure we kill the entire process tree using the tree-kill npm package (tree-kill is something thats already used)

[DonJayamanne]

Option 1:

• After DA shutsdown, call treekill on the parent process.
• We already do this, & as we have users reporting issues, this means only one thing - its not working. After all, the parent process is already dead (killed by PTVSD itself - in response to shutdown request)!

Option 2:

• TS DA, intercepts all messages and acts as a proxy. Today the TA DA listens to messages, it does not intercept it.
• Once a shutdown request is received, TS DA could then decide to manually kill the python process using treekill, and then send the other DA messages to VSC (messages that would generally be sent back by PTVSD).

Option 3:

• PTVSD does all of this.

[DonJayamanne]

Prescribed solution:

• Use psutils to kill process tree