Add Slave and Framework struct to HierarchicalAllocatorProcess. Cleans
up the proliferation of hashmaps in HierarchicalAllocatorProcess.

From: Thomas Marshall <twm378@gmail.com>
Review: https://reviews.apache.org/r/11541
Added a retry option to cgroups::mount in order to deal with a bug
with the kernel unmounting the hierarchy asynchronously causing
subsequent mounts to fail.

From: Thomas Marshall <twm378@gmail.com>
Review: https://reviews.apache.org/r/11547