pool: Restore flush and stage stats in info

Resolves a regression in which statistics about the number of HSM
requests and HSM timeouts is no longer provided in the info output.

Also fixes a minor bug in how active HSM jobs are counted: A cancelled
task is still active until the driver has ended the job.

Target: trunk
Request: 2.11
Request: 2.10
Request: 2.9
Require-notes: yes
Require-book: yes
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: https://rb.dcache.org/r/7520/