rbd-mirror: address compiler warnings

rbd-mirror: address compiler warnings

Issue I:
src/tools/rbd_mirror/group_replayer/Replayer.cc:604:82:
error: overlapping comparisons always evaluate to true [-Werror,-Wtautological-overlap-compare]
  604 | (prev_remote_snap_ns->state != cls::rbd::MIRROR_SNAPSHOT_STATE_PRIMARY ||
      |  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~
  605 |  prev_remote_snap_ns->state != cls::rbd::MIRROR_SNAPSHOT_STATE_PRIMARY_DEMOTED)) {
      |  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Issue II:
src/tools/rbd_mirror/GroupReplayer.h:178:10:
error: lambda capture 'this' is not used [-Werror,-Wunused-lambda-capture]
  178 |         [this](int r) {
      |          ^

Issue III:
src/test/rbd_mirror/test_mock_ImageSync.cc:258:16:
error: no matching constructor for initialization of 'MockImageSync'
(aka 'ImageSync<librbd::MockTestImageCtx>')
  258 |     return new MockImageSync(

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>