Introduce ProjectManager class to manage MSBuild project files

This is a pretty substantial change that reworks the core logic of the MSBuild project
system to enable an important scenario: updating a project when several files change in quick
succession.

In order to fix an issue with OmniSharp not reloading and updating a project in response to
a 'dotnet restore', we must watch four files that might be touched during a restore:

* project.asset.json
* <project-file>.nuget.cache
* <project-file>.nuget.g.props
* <project-file>.nuget.g.targets

To ensure that we don't reload and update a project multiple times in response to multiple file
changes, this PR introduces a simple queue and processing loop using a TPL DataFlow BufferBlock.

src/OmniSharp.Abstractions/Models/v1/FilesChanged/FileChangeType.cs → src/OmniSharp.Abstractions/FileWatching/FileChangeType.cs

@@ -1,4 +1,4 @@
-namespace OmniSharp.Models.FilesChanged
+namespace OmniSharp.FileWatching
 {
     public enum FileChangeType
     {

src/OmniSharp.Abstractions/Models/v1/FilesChanged/FilesChangedRequest.cs

@@ -1,4 +1,5 @@
 using System.Collections.Generic;
+using OmniSharp.FileWatching;
 using OmniSharp.Mef;
 
 namespace OmniSharp.Models.FilesChanged