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/CancelTokenKey.cs

@@ -0,0 +1,36 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Microsoft.SqlTools.ServiceLayer.Connection.Contracts;
+
+namespace Microsoft.SqlTools.ServiceLayer.Connection
+{
+    /// <summary>
+    /// Used to uniquely identify a CancellationTokenSource associated with both
+    /// a string URI and a string connection type.
+    /// </summary>
+    public class CancelTokenKey : CancelConnectParams, IEquatable<CancelTokenKey>
+    {
+        public override bool Equals(object obj)
+        {
+            CancelTokenKey other = obj as CancelTokenKey;
+            if (other == null)
+            {
+                return false;
+            }
+
+            return other.OwnerUri == OwnerUri && other.Type == Type;
+        }
+
+        public bool Equals(CancelTokenKey obj)
+        {
+            return obj.OwnerUri == OwnerUri && obj.Type == Type;
+        }
+
+        public override int GetHashCode()
+        {
+            return OwnerUri.GetHashCode() ^ Type.GetHashCode();
+        }
+    }
+}