SWDEV-520352 - Remove HostThread and legacy monitor (#230)

* SWDEV-520352 - Remove HostThread and legacy monitor

Remove HostThread, semaphore and  legacy monitor.
Make original logics of thread and command queue stricker.
Add more comments to make logics clearer.
Some other minor improvement.

Also part of SWDEV-458943.

rocclr/platform/commandqueue.cpp

@@ -51,7 +51,10 @@ HostQueue::HostQueue(Context& context, Device& device, cl_command_queue_properti
     if (thread_.state() >= Thread::INITIALIZED) {
       ScopedLock sl(queueLock_);
       thread_.start(this);
-      queueLock_.wait();
+      // wait for HostQueue::loop() to update acceptingCommands_ as true
+      while (!thread_.acceptingCommands_) {
+        queueLock_.wait();
+      }
     }
   }
 }
@@ -204,6 +207,7 @@ void HostQueue::loop(device::VirtualDevice* virtualDevice) {
   // Notify the caller that the queue is ready to accept commands.
   {
     ScopedLock sl(queueLock_);
+    // Notify HostQueue() that acceptingCommands_ is updated to true
     thread_.acceptingCommands_ = true;
     queueLock_.notify();
   }