RFR: Make weakref processing always use Executor

[Aleksey Shipilev]

On 08/10/2018 06:47 PM, Roman Kennke wrote:
> Incremental:
> http://cr.openjdk.java.net/~rkennke/weakrefs-1thread/webrev.01.diff/
> Full:
> http://cr.openjdk.java.net/~rkennke/weakrefs-1thread/webrev.01/

OK. It is still ugly-ish to always go to executor to avoid our own oom-evac-scope bugs, but oh well.

-Aleksey