cleanup.
* made BMessageQueue::IsLocked() const - the non-const version is still provided
for binary compatibility.
* Both BMessageQueue::FindMessage() versions are now thread safe, the queue's BLocker
is now mutable to allow for this.
* renamed BMessage::link to fQueueLink as the "Message4" implementation uses it.
git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@14955 a95241bf-73f2-0310-859d-f6bbb57e9c96
