Prevent flowing ExecutionContext

Prevent flowing the Execution Context to background threads. Without this, it means values from AsyncLocal and CallContext will flow/leak into background threads which should be avoided because it may inadvertently be capturing some ambient contexts like DbContext, etc... which can have unwanted side affects. In most cases anything stored in AsyncLocal is also not thread safe and if it's a mutable object it means concurrent threads can be modifying it which also may cause unwanted side affects. I don't think there's a use-case for flowing the execution context into a background thread since the execution in these threads should be entirely isolated.

I put together a gist showing all the different ways the AsyncLocal/CallContext values flow to child threads here https://gist.github.com/Shazwazza/c127e8c567ab505d146e54ac802a9945

I think this is the only place where the created threads are actually started but I could be wrong. Calling SuppressFlow is only required where the thread is started.

src/Hangfire.Core/Processing/BackgroundTaskScheduler.cs

@@ -131,10 +131,21 @@ public BackgroundTaskScheduler(
                 throw new ArgumentException("All the threads should be non-null and in the ThreadState.Unstarted state.", nameof(threadFactory));
             }
 
-            foreach (var thread in _threads)
+            // Prevent flowing the Execution Context to background threads.
+            // Without this, it means values from AsyncLocal and CallContext
+            // will flow/leak into background threads which should be avoided
+            // because it may inadvertently be capturing some ambient contexts
+            // like DbContext, etc... which can have unwanted side affects.
+            // In most cases anything stored in AsyncLocal is also not thread
+            // safe and if it's a mutable object it means concurrent threads can
+            // be modifying it which also may cause unwanted side affects.
+            using (ExecutionContext.SuppressFlow())
             {
-                thread.Start();
-            }
+                foreach (var thread in _threads)
+                {
+                    thread.Start();
+                }
+            }            
 
             _ourThreadIds = new HashSet<int>(_threads.Select(x => x.ManagedThreadId));
         }