Memo: dotnet restore improvement

Design

1. User opens a new cli solution in editor. The projects in this solution are not restored.
2. The OmniSharp invoke dotnet project system to parse the projects.
3. The dotnet project system (DPS) returns diagnostics indicate these projects need to be restored.
4. The restore requests are pooled together. Duplicate requests on same project are distinct and then restore options are carried out in parallel (or none parallel depending on the settings)
5. During restoring, events are sent to editor to indicate the progress.
6. After restoring, the DPS is requested to solve dependencies again. All unresolved dependencies will be gathered and send to editor.