Connect with different properties should actually change context (#307)

* Connect with different properties should actually change context
- Up to now, calling Connect for a previously-connected URI would disconnect, then reconnect ot the original (not new) target. WIth these changes we handle changes to database name or other key properties by updating the ConnectionInfo and connecting to the new target
- Some interesting scenarios are raised by our API, notably that an empty database name maps to the default DB (which we know nothing about). This limits the new feature such that only if the DB Name is specified, we'll change the connection. Hence 2 calls to an empty DB will not result in a DB change.

Additional changes:
- After discussion with Ben, we're simplifying the cancellation logic. He had made changes to support this, so the main update is that we dispose the token in the final block after its last use (hence avoiding a disposed exception) and clean up the number of Waits required since we already have async cancellation support
- Factored some logic such that the OnConnection callback isn't invoked until after we've updated the database name in the GetConnectionCompleteParams method. Again, this supports reporting the actual DB name instead of leaving it blank for default DB requests.

* PR comment fixes

src/Microsoft.SqlTools.ServiceLayer/Connection/ConnectionInfo.cs

@@ -99,6 +99,12 @@ namespace Microsoft.SqlTools.ServiceLayer.Connection
             }
         }
 
+        public bool HasConnectionType(string connectionType)
+        {
+            connectionType = connectionType ?? ConnectionType.Default;
+            return ConnectionTypeToConnectionMap.ContainsKey(connectionType);
+        }
+
         /// <summary>
         /// The count of DbConnectioninstances held by this ConnectionInfo 
         /// </summary>
@@ -155,7 +161,6 @@ namespace Microsoft.SqlTools.ServiceLayer.Connection
                 DbConnection connection;
                 ConnectionTypeToConnectionMap.TryRemove(type, out connection);
             }
-        }
-
+        } 
     }
 }