librbd: support reading multiple extents in the object dispatch interface

librbd: support reading multiple extents in the object dispatch interface

This commit extends the object dispatch read function to support multiple object extents (using a single librados ObjectOperation).
In addition, we add an option to return the version number for that read.
This new function will be used by the new crypto object dispatch layer to support read-modify-write operation required for encrypting and writing unaligned data.

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