crimson/osd/pg: SnapTrimEvent to support interrupts

crimson/osd/pg: SnapTrimEvent to support interrupts

SnapTrimEvent operations are scheduled from `PG::on_active_actmap()`
using a `seastar::do_until` loop. This commit replaces the loop type
into an `interruptor::repeat` and SnapTrimEvent are now scheduled by
`start_operation_may_interrupt`.

Previously, `SnapTrimEvent::start` handled interruptions by returning
a `crimson::ct_error::eagain::make();`. Now, the errorator is directly
returned via the `snap_trim_event_ret_t` and interrupts the loop
described above.

As a result, interruptions originated by interval changes are now
supported by SnapTrimEvent.

Signed-off-by: Xuehan Xu <xuxuehan@qianxin.com>