Move includes of events.hpp out of header files

I was making changes to events.hpp and saw that the majority of cpp files were recompiled. There are two header files that include events.hpp. I propose that we remove those usages by:

jvmci.hpp: Use a forward declaration
collectedHeap.hpp: Move its usage to the cpp file

events.hpp also includes thread.hpp, which brings in a large portion of the HotSpot headers. So, this change is also good in that regard.