crimson/osd/osd_operations/snaptrim_event: lifetime fixes

crimson/osd/osd_operations/snaptrim_event: lifetime fixes

```
    // SnapTrimEvent is a background operation,
    // it's lifetime is not guarnteed since the caller
    // returned future is being ignored. We should capture
    // a self reference thourhgout the entire execution
    // progress (not only on finally() continuations).
    // See: PG::on_active_actmap()
```

Sanitized backtrace:
```
DEBUG 2023-11-16 08:42:48,441 [shard 0] osd - snaptrim_event(id=21122, detail=SnapTrimEvent(pgid=3.1 snapid=3cb needs_pause=1)): interrupted crimson::common::actingset_changed (acting set changed

kernel callstack:
    #0 0x55e310e0ace7 in seastar::shared_mutex::unlock() (/usr/bin/ceph-osd+0x1edd0ce7)
    #1 0x55e313325d9c in auto seastar::futurize_invoke<crimson::OrderedConcurrentPhaseT<crimson::osd::SnapTrimEvent::WaitSubop>::ExitBarrier<crimson::OrderedConcurrentPhaseT<crimson::osd::SnapTrimEvent::WaitSubop>::BlockingEvent::Trigger<crimson::osd::SnapTrimEvent> >::exit()::{lambda()#1}&>(crimson::OrderedConcurrentPhaseT<crimson::osd::SnapTrimEvent::WaitSubop>::ExitBarrier<crimson::OrderedConcurrentPhaseT<crimson::osd::SnapTrimEvent::WaitSubop>::BlockingEvent::Trigger<crimson::osd::SnapTrimEvent> >::exit()::{lambda()#1}&) (/usr/bin/ceph-osd+0x212ebd9c)
    #2 0x55e3133260ef in _ZN7seastar20noncopyable_functionIFNS_6futureIvEEvEE17direct_vtable_forIZNS2_4thenIZN7crimson23OrderedConcurrentPhaseTINS7_3osd13SnapTrimEvent9WaitSubopEE11ExitBarrierINSC_13BlockingEvent7TriggerISA_EEE4exitEvEUlvE_S2_EET0_OT_EUlDpOT_E_E4callEPKS4_ (/usr/bin/ceph-osd+0x212ec0ef)
0x61500013365c is located 92 bytes inside of 472-byte region [0x615000133600,0x6150001337d8)
freed by thread T2 here:
    #0 0x7fb345ab73cf in operator delete(void*, unsigned long) (/lib64/libasan.so.6+0xb73cf)
    #1 0x55e313474863 in crimson::osd::SnapTrimEvent::~SnapTrimEvent() (/usr/bin/ceph-osd+0x2143a863)

previously allocated by thread T2 here:
    #0 0x7fb345ab6367 in operator new(unsigned long) (/lib64/libasan.so.6+0xb6367)
    #1 0x55e31183ac18 in auto crimson::OperationRegistryI::create_operation<crimson::osd::SnapTrimEvent, crimson::osd::PG*, SnapMapper&, snapid_t const&, bool const&>(crimson::osd::PG*&&, SnapMapper&, snapid_t const&, bool const&) (/usr/bin/ceph-osd+0x1f800c18)
SUMMARY: AddressSanitizer: heap-use-after-free (/usr/bin/ceph-osd+0x1edd0ce7) in seastar::shared_mutex::unlock()
```

Signed-off-by: Matan Breizman <mbreizma@redhat.com>