Make _MultiHandle timeout timer non-repeatable

CURL documentation (https://curl.se/libcurl/c/CURLMOPT_TIMERFUNCTION.html)
explicitly says that the timer should be one-time. We basically have to
follow CURL requests for setting, resetting and disarming such timers.

Current logic eventually leaves a 1ms repeating timer forever, because
CURL assumes it fires once, and may not ask us to remove it explicitly.

Also, being used as request timeout trigger, this timer also has no sense
to be repeated.

Sources/FoundationNetworking/URLSession/libcurl/MultiHandle.swift

@@ -359,7 +359,7 @@ class _TimeoutSource {
         let delay = UInt64(max(1, milliseconds - 1)) 
         let start = DispatchTime.now() + DispatchTimeInterval.milliseconds(Int(delay))
 
-        rawSource.schedule(deadline: start, repeating: .milliseconds(Int(delay)), leeway: (milliseconds == 1) ? .microseconds(Int(1)) : .milliseconds(Int(1)))
+        rawSource.schedule(deadline: start, repeating: .never, leeway: (milliseconds == 1) ? .microseconds(Int(1)) : .milliseconds(Int(1)))
         rawSource.setEventHandler(handler: handler)
         rawSource.resume() 
     }
@@ -384,13 +384,12 @@ fileprivate extension URLSession._MultiHandle {
             timeoutSource = nil
             queue.async { self.timeoutTimerFired() }
         case .milliseconds(let milliseconds):
-            if (timeoutSource == nil) || timeoutSource!.milliseconds != milliseconds {
-                //TODO: Could simply change the existing timer by using DispatchSourceTimer again.
-                let block = DispatchWorkItem { [weak self] in
-                    self?.timeoutTimerFired()
-                }
-                timeoutSource = _TimeoutSource(queue: queue, milliseconds: milliseconds, handler: block)
+            //TODO: Could simply change the existing timer by using DispatchSourceTimer again.
+            let block = DispatchWorkItem { [weak self] in
+                self?.timeoutTimerFired()
             }
+            // Note: Previous timer instance would cancel internal Dispatch timer in deinit
+            timeoutSource = _TimeoutSource(queue: queue, milliseconds: milliseconds, handler: block)
         }
     }
     enum _Timeout {