[profiler] Move HEAP_START and HEAP_EVENTS to happen outside of heap shots.

[profiler] Move HEAP_START and HEAP_EVENTS to happen outside of heap shots.

They now are produced every time a GC spills heap events such as roots, moves and objects.

Additionally, I changed GC syncpoints to happen in case any GC events are active.
It's a safer choice with little penalty. Figuring out the actual combinations that would require them is non-trivial.