Core: make MtProtoSender.GenerateSequence() thread-safe

Even if you're not supposed to use multiple threads with TLSharp,
it might be worth it to try to make this sequence increment in
a thread-safe way. Race conditions are always bad even if you know
you are not supposed to use something in a certain way...

@Laituex was having a ErrCode=32 'msg_seqno too low' problem and this
is the first thing we looked at (even if he swore that he was not
using different threads to access the telegram network)

TLSharp.Core/Network/MtProtoSender.cs

@@ -34,8 +34,10 @@ namespace TLSharp.Core.Network
 
         private int GenerateSequence(bool confirmed)
         {
+            lock (session.Lock) {
                 return confirmed ? session.Sequence++ * 2 + 1 : session.Sequence * 2;
             }
+        }
 
         public async Task Send(TeleSharp.TL.TLMethod request, CancellationToken token = default(CancellationToken))
         {

TLSharp.Core/Session.cs

@@ -72,6 +72,8 @@ namespace TLSharp.Core
 
     public class Session
     {
+        internal object Lock = new object ();
+
 	    private const string defaultConnectionAddress = "149.154.175.100";//"149.154.167.50";
 
 		private const int defaultConnectionPort = 443;