CA-141945: Add reporting of TRIM usage to SR

Once a TRIM successfully completed, note the time it was run on the SR for
reporting purposes.
Initially useful for TaaS statistics to show when features have been used.

Signed-off-by: Bob Ball <bob.ball@citrix.com>
Reviewed-by: Robert Breker <robert.breker@citrix.com>
Reviewed-by: Mate Lakat <mate.lakat@citrix.com>
Reviewed-by: Germano Percossi <germano.percossi@citrix.com>

GitHub: closes #200 on xapi-project/sm