Fixed a crash when metrics race with quota removal.
Fixed a crash when metrics race with quota removal.

This patch addresses a race condition in which the removal of
a role from the allocator's quota sorter races with execution
of a callback tied to a `PullGauge`. The gauge's callback
assumed that the role would be present in the sorter, but it's
possible for the role to be removed before the callback is
executed.

Review: https://reviews.apache.org/r/67104/

abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,reneploetz/mesos,gsantovena/mesos,andschwa/mesos,chhsia0/mesos,kaysoky/mesos,reneploetz/mesos,chhsia0/mesos,chhsia0/mesos,abudnik/mesos,gsantovena/mesos,jpeach/mesos,Gilbert88/mesos,gsantovena/mesos,dforsyth/mesos,jpeach/mesos,kaysoky/mesos,chhsia0/mesos,abudnik/mesos,abudnik/mesos,kaysoky/mesos,Gilbert88/mesos,Gilbert88/mesos,Gilbert88/mesos,abudnik/mesos,dforsyth/mesos,jpeach/mesos,gsantovena/mesos,dforsyth/mesos,andschwa/mesos,jpeach/mesos,reneploetz/mesos,jpeach/mesos,andschwa/mesos,dforsyth/mesos,kaysoky/mesos,andschwa/mesos,kaysoky/mesos,gsantovena/mesos,reneploetz/mesos,dforsyth/mesos,gsantovena/mesos,reneploetz/mesos,chhsia0/mesos,dforsyth/mesos,dforsyth/mesos,kaysoky/mesos,Gilbert88/mesos,andschwa/mesos,abudnik/mesos,gsantovena/mesos,jpeach/mesos,chhsia0/mesos,chhsia0/mesos,jpeach/mesos,andschwa/mesos,Gilbert88/mesos,andschwa/mesos,reneploetz/mesos,abudnik/mesos,reneploetz/mesos