Add IsParting() flag to track PART commands waiting IRCd response.

Fixes potential desync where ZNC marks channel as parted before server
confirms the PART, which could cause issues if the server never replies.

Add unittests and integration tests.

src/Client.cpp

@@ -1469,7 +1469,7 @@ bool CClient::OnJoinMessage(CJoinMessage& Message) {
             if (pChan) {
                 if (pChan->IsDetached())
                     pChan->AttachUser(this);
-                else
+                else if (!pChan->IsOn() || pChan->IsParting())
                     pChan->JoinUser(sKey);
                 continue;
             } else if (!sChannel.empty()) {
@@ -1592,7 +1592,7 @@ bool CClient::OnPartMessage(CPartMessage& Message) {
             PutStatusNotice(t_f("Removing channel {1}")(sChan));
             m_pNetwork->DelChan(sChan);
         } else {
-            if (pChan) pChan->SetIsOn(false);
+            if (pChan) pChan->SetParting(true);
             sChans += (sChans.empty()) ? sChan : CString("," + sChan);
         }
     }