Merge pull request #5645 from xinxinsh/wip-snap-rename

implement snapshot rename

Reviewed-by: Jason Dillaman <dillaman@redhat.com>

diff --cc src/librbd/ImageCtx.cc
Simple merge

diff --cc src/librbd/ImageWatcher.cc
Simple merge

diff --cc src/librbd/internal.cc
Simple merge

diff --cc src/librbd/librbd.cc
Simple merge

diff --cc src/rbd.cc
Simple merge

diff --cc src/test/librbd/test_librbd.cc
Simple merge