common/bl: introduce rvalue taking claim_append().

common/bl: introduce rvalue taking claim_append().

The motivation is to stop prohibiting callers from
directly passing temporaries. It was not infrequent
to see constructs like:

  ```cpp
  {
    // ...
    auto tmp = produce_bl();
    other_bl.claim_append(tmp);
  }
  ```

They were necessary because `claim_append(bufferlist&)`
requires lvalue reference to which an rvalue can't bind.

This patch fixes the problem by introducing `&&`-taking
variant of `claim_append()`. In the future it could
actually supersede the original one.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>