Add scripting API implemented by the SqlScriptPublishModel (#316)

Update the ScriptingService to expose new scripting JSON-RPC APIs that use the SqlScriptPublishModel for script generation.

The SqlScriptPublishModel is the model behind the SSMS scripting wizard. To enable scripting for CLI tools, we've ported SqlScriptPublishModel to .NET Core. The SqlScriptPublishModel wraps the SMO scripting APIs for .sql script generation.

1) Added three new requests to the ScriptingService: ScriptingRequest, ScriptingListObjectsRequest, ScriptingCancelRequest.
2) Generating scripts are long running operations, so the ScriptingRequest and ScriptingListObjectsRequest kick off a long running scripting task and return immediately.
3) Long running scripting task reports progress and completion, and can be cancelled by a ScriptingCancelRequest request.
4) Bumped the SMO nuget package to 140.17049.0. This new version contains a signed SSMS_Rel build of SMO with the SqlScriptPublishModel.
5) For testing, adding the Northwind database schema

TODO (in later pull requests)
1) Integrate the new ScriptingService APIs with the ConnectionService
2) Integrate with the metadata support recently added

test/Microsoft.SqlTools.ServiceLayer.UnitTests/project.json

@@ -19,7 +19,7 @@
     "System.Runtime.Serialization.Primitives": "4.1.1",
     "System.Data.Common": "4.1.0",
     "System.Data.SqlClient": "4.4.0-sqltools-24613-04",
-    "Microsoft.SqlServer.Smo": "14.0.17028",
+    "Microsoft.SqlServer.Smo": "140.17049.0",
     "System.Security.SecureString": "4.0.0",
     "System.Collections.Specialized": "4.0.1",
     "System.ComponentModel.TypeConverter": "4.1.0",