librbd: add crypto image dispatch layer

librbd: add crypto image dispatch layer

This commit adds a crypto image dispatch layer, in addition to the existing crypto object dispatch layer.
The single purpose of this layer is to translate logical image offset to "on-disk" offsets,
given a crypto header which occupies the beginning of the image data.
This calculation is done via a new remap_extents api,
which is used in to wrap existing Striper translations between object and image extents.

Signed-off-by: Or Ozeri <oro@il.ibm.com>