Debugger: Processes 'dotnet' and 'clrdbg' are still running after stopping debugging.

## Environment data
`dotnet --info` output:
VS Code version: 1.9.1
C# Extension version: 1.7.0
dotnet cli: 1.0.0-preview2-1-003177
OS: Windows 10 Enterprise Insider Preview
Build: 15032.1000.AMD64FRE.RS_ONECORE_BASE_BPT1.170207-1700_CLIENT_ENTERPRISE_EN-US_VL.VHDX
Scenarios: WSL Pipe

## Steps to reproduce
1. Open attached folder
[app2.zip](https://github.com/OmniSharp/omnisharp-vscode/files/770504/app2.zip)

2. Set a bp at the line of 12 (i++).
3. Press F5 to start debugging.
4. Remove the bp.
5. Press F5 to continue debugging.
6. Press Pause(F6) to break all. (The 'break all' has no effect )
7. Stop debugging

## Expected behavior
1. The 'break all'(#5) should work correctly.
2. Processes 'dotnet' and 'clrdbg' have stopped after stop debugging.

## Actual behavior
1. The 'break all' has no effect
2. Processes 'dotnet' and 'clrdbg' are still running after stopping debugging.

## Investigation
1. After the above steps, set a bp and continue F5 to start debugging, the debugging will become very slow.