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So we can parallelise minor collections too.  Sometimes it's worth it.

DEBUG imposes a significant performance hit in the GC, yet we often
want some of the debugging output, so -vg gives us the cheap trace
messages without the sanity checking of DEBUG, just like -vs for the
scheduler.

When a stack is occupying less than 1/4 of the memory it owns, and is
larger than a megablock, we release half of it.  Shrinking is O(1), it
doesn't need to copy the stack.

There was already a check to avoid updating an IND, but it was
originally there to avoid a bug which doesn't exist now.  Furthermore
the test and update are not atomic, so another thread could be
updating this thunk while we are.  We have to just go ahead and update
anyway - it might waste a little work, but this is a very rare case.

in addition to checking for leaks