Ensure connection open for OE queries and fix connection disposal (#359)

* Ensure connection open for OE queries and fix connection disposal
- Dispose connection in Metadata service, to ensure we cleanly dispose and don't rely on garbage colleciton
- Fixed issue where if the connection was closed, expanding databases in the Server would fail. This is because SMO doesn't always reopen the connection, certainly not for Server level queries. The solution is to always check if open and reopen.
- Added unit tests for this, which required mocking the relevant IsOpen / OpenConnection methods. Refactored SMO wrapper calls into a dedicated class file to handle this

src/Microsoft.SqlTools.ServiceLayer/ObjectExplorer/SmoModel/SmoQuerier.cs

@@ -92,7 +92,10 @@ namespace Microsoft.SqlTools.ServiceLayer.ObjectExplorer.SmoModel
                     Enumerator en = new Enumerator();
                     Request request = new Request(new Urn(urn));
                     ServerConnection serverConnection = new ServerConnection(context.Server.ConnectionContext.SqlConnectionObject);
-
+                    if (!serverConnection.IsOpen)
+                    {
+                        serverConnection.Connect();
+                    }
                     EnumResult result = en.Process(serverConnection, request);
 
                     urns = GetUrns(result);