RFR: Refactor allocation failure and explicit GC handling

[Aleksey Shipilev]

On 01/17/2018 03:45 AM, Zhengyu Gu wrote:
> ShenandoahConcurrentThread::handle_alloc_failure() now takes monitor lock, and this method is in
> ShenandoahHeap::allocate_memory() path, in turn, can be called inside write barrier ... seems to be
> the scenario we talked before, that we *can not* do.

Note that allocate_memory on the *shared/TLAB* allocation path was taking a lock in the old code
too: see the path in ShHeap::allocate_memory -> ShHeap::collect(_allocation_failure) ->
ShConcThread::do_full_gc. The trick here is not to lock when shared_gc/GCLAB allocation fails, and
this is why we have separate ::handle_alloc_failure_evac().

Thanks,
-Aleksey