Update connection logic to handle multiple connections per URI (#176)

* Add CancelTokenKey for uniquely identifying cancelations of Connections associated with an OwnerUri and ConnectionType string.

* Update ConnectionInfo to use ConcurrentDictionary of DbConnection instances. Add wrapper functions for the ConcurrentDictionary. 

* Refactor Connect and Disconnect in ConnectionService.

* Update ConnectionService: Handle multiple connections per ConnectionInfo. Handle cancelation tokens uniquely identified with CancelTokenKey. Add GetOrOpenConnection() for other services to request an existing or create a new DbConnection.

* Add ConnectionType.cs for ConnectionType strings.

* Add ConnectionType string to ConnectParams, ConnectionCompleteNotification, DisconnectParams.

* Update Query ExecuteInternal to use the dedicated query connection and GetOrOpenConnection().

* Update test library to account for multiple connections in ConnectionInfo.

* Write tests ensuring multiple connections don’t create redundant data.

* Write tests ensuring database changes affect all connections of a given ConnectionInfo.

* Write tests for TRANSACTION statements and temp tables.

src/Microsoft.SqlTools.ServiceLayer/Connection/Contracts/DisconnectParams.cs

@@ -15,5 +15,11 @@ namespace Microsoft.SqlTools.ServiceLayer.Connection.Contracts
         /// or a virtual file representing an object in a database.         
         /// </summary>
         public string OwnerUri { get; set; }
+
+        /// <summary>
+        /// The type of connection we are disconnecting. If null, we will disconnect all connections.
+        /// connections. 
+        /// </summary>
+        public string Type { get; set; }
     }
 }