We are investigating what it would take to remove PerThreadCache and
refactor the rest of the default permission backend.
One potential cause of slowness and the success of PerThreadCache are
two access computations, one in ProjectCache, one on
PermissionCollection.
This commit instruments these to validate our assumption. The metrics
will be removed once we have data.
Change-Id: I5f39ae7113d657bbb69b693367aaa0930cf21900
f71bafe885