crimson/osd: cancel IO reservations on PG::stop().

`PG::request_{local,remote}_recovery_reservation()` dynamically allocates
up to 2 instances of `LambdaContext<T>` and transfers their ownership to
the `AsyncReserver<T, F>`. This is expressed in raw pointers (`new` and
`delete`) notion. Further analysis shows the only place where `delete`
for these objects is called is the `AsyncReserver::cancel_reservation()`.
In contrast to the classical OSD, crimson doesn't invoke the method when
stopping a PG during the shutdown sequence. This would explain the
following ASan issue observed at Sepia:

```
Direct leak of 576 byte(s) in 24 object(s) allocated from:
    #0 0x7fa108fc57b0 in operator new(unsigned long) (/lib64/libasan.so.5+0xf17b0)
    #1 0x55723d8b0b56 in non-virtual thunk to crimson::osd::PG::request_local_background_io_reservation(unsigned int, std::unique_ptr<PGPeeringEvent, std::default_delete<PGPeeringEvent> >, std::unique_ptr<PGPeeringEvent, std::default_delete<PGPeeringEvent> >) (/usr/bin/ceph-osd+0x24d95b56)
    #2 0x55723f1f66ef in PeeringState::WaitDeleteReserved::WaitDeleteReserved(boost::statechart::state<PeeringState::WaitDeleteReserved, PeeringState::ToDelete, boost::mpl::list<mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na, mpl_::na>, (boost::statechart::history_mode)0>::my_context) (/usr/bin/ceph-osd+0x266db6ef)
```

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>

src/crimson/osd/pg.cc

@@ -1153,6 +1153,8 @@ seastar::future<> PG::stop()
 {
   logger().info("PG {} {}", pgid, __func__);
   stopping = true;
+  cancel_local_background_io_reservation();
+  cancel_remote_recovery_reservation();
   check_readable_timer.cancel();
   renew_lease_timer.cancel();
   return osdmap_gate.stop().then([this] {