[qjs] Add ability to remove handlers.

The current code will just keep adding handlers forever.

Test code is only possible if we have a way of manipulating handles
from JS.  This functionality is all in the Fuchsia layer; any test
that uses handles and runs a loop will exercise this, because without
it, the loop will run forever.  Such a test will be checked in as
part of the "list directories" CL.

Change-Id: I41d8e976c9c8abce6c53d5efb051c4d66c13aab8
2 files changed