Debugger: takes a long time to start and populate the locals window for the first time

## Environment data
`dotnet --info` output:
VS Code version: 1.7.1
C# Extension version: v.1.5.0-beta7
.NET Core version: 1.0.0-SDK Preview4

## Steps to reproduce

1. Open hwapp folder in attachments directly
2. Set a breakpoint at line of 10 in Program.cs file
3. Press F5 to start debugging

## Expected behavior
It should display immediately variables value in local window

## Actual behavior
After it hit breakpoint ,and it display variables value more than 6 seconds in local window
[hwapp.zip](https://github.com/OmniSharp/omnisharp-vscode/files/580362/hwapp.zip)