ceph_test_objectstore: fix Synthetic to never modify bufferlists

We were modifying bufferlists in place, and kludging around it by making
full copies elsewhere.  Instead, never modify a buffer.

This fixes issues where the buffer we submit to ObjectStore ends up in
the cache and we modify in place later, corrupting the implementation's
copy.  (This was affecting BlueStore.)

Rearrange the data methods to be next to each other and clean them up a
bit too.

Signed-off-by: Sage Weil <sage@redhat.com>