Slow memory growth in master due to deferred deletion of offer filters and timers.

The allocator does not keep a handle to the offer filter timer, which means it cannot remove the timer overhead (in this case memory) when removing the offer filter earlier (e.g. due to revive):

https://github.com/apache/mesos/blob/1.8.0/src/master/allocator/mesos/hierarchical.cpp#L1338-L1352

In addition, the offer filter is allocated on the heap but not deleted until the timer fires (which might take forever!):

https://github.com/apache/mesos/blob/1.8.0/src/master/allocator/mesos/hierarchical.cpp#L1321
https://github.com/apache/mesos/blob/1.8.0/src/master/allocator/mesos/hierarchical.cpp#L1408-L1413
https://github.com/apache/mesos/blob/1.8.0/src/master/allocator/mesos/hierarchical.cpp#L2249

We'll need to try to backport this to all active release branches.