hints: assign _last_written_rp in ep manager's move constructor
hints: assign _last_written_rp in ep manager's move constructor

The end_point_hints_manager's field _last_written_rp is initialized in
its regular constructor, but is not copied in the move constructor.
Because the move constructor is always involved when creating a new
endpoint manager, the _last_written_rp field is effectively always
initialized with the zero-argument constructor, and is set to the zero
value.

This can cause the following erroneous situation to occur:

- Node A accumulates hints towards B.
- Sync point is created at A.
  It will be used later to wait for currently accumulated hints.
- Node A is restarted.
  The endpoint manager A->B is created which has bogus value in the
  _last_written_rp (it is set to zero).
- Node A replays its hints but does not write any new ones.
- A hint flush occurs.
  If there are no hint segments on disk after flush, the endpoint
  manager sets its last sent position to the last written position,
  which is by design. However, the last written position has incorrect
  value, so the last sent position also becomes incorrect and too low.
- Try to wait for the sync point created earlier.
  The sync point waiting mechanism waits until last sent hint position
  reaches or goes past the position encoded in the sync point, but it
  will not happen because the last sent position is incorrect.

The above bug can be (sometimes) reproduced in
hintedhandoff_sync_point_api_test dtest.

Now, the _last_written_rp field is properly initialized in the move
constructor, which prevents the bug described above.

Fixes: #9320
Closes #9426

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla