[mssql] Don't share connections between threads

Starting with Qt 5.11, sharing the same connection between
threads is not allowed. Use a dedicated connection for each
thread requiring access to the database, using the thread
address as connection name.

https://bugreports.qt.io/browse/QTBUG-68486 indicates that
this change is by design and that sharing connections between
threads was always unsafe.

Fixes broken mssql provider on Qt 5.11, and possibly who know
what issues on earlier Qt.

Ported from mumble-voip/mumble@a3814e33

src/providers/mssql/qgsmssqlprovider.cpp

@@ -233,6 +233,12 @@ QSqlDatabase QgsMssqlProvider::GetDatabase( const QString &service, const QStrin
   else
     connectionName = service;
 
+  // Starting with Qt 5.11, sharing the same connection between threads is not allowed.
+  // We use a dedicated connection for each thread requiring access to the database,
+  // using the thread address as connection name.
+  const QString threadAddress = QStringLiteral( ":0x%1" ).arg( reinterpret_cast< quintptr >( QThread::currentThreadId() ), 16 );
+  connectionName += threadAddress;
+
   if ( !QSqlDatabase::contains( connectionName ) )
   {
     db = QSqlDatabase::addDatabase( QStringLiteral( "QODBC" ), connectionName );