Objecter, librados: use only ObjectOperation form of sparse_read internally

Objecter, librados: use only ObjectOperation form of sparse_read internally

This will be used when exposing an ObjectOperation version of sparse_read()
to the librados user, and there's no reason to duplicate code for creating
and handling it. Add a wrapper Context for handling the lifecycle of the
::ObjectOperation.

This cleans up the synchronous version of sparse_read quite a bit by
using the general operate_read() instead of duplicating decoding and
a bunch of sync boilerplate.

Move handling the decoding of a sparse_read into the Objecter, with
the rest of the decoding of rados operations. librados shouldn't be
the only user of the Objecter that can understand sparse_reads.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>