ObjectStore: add bufferlist/string getattr

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: fix size checking for ECBackend pools

Allow the backend to adjust the logical object size before
checking against the ondisk size.

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd/: add the length to the ObjectModDesc append operation

Signed-off-by: Samuel Just <sam.just@inktank.com>
ECUtil: remove per-chunk checksums, add HashInfo

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGBackend: make getattrs virtual

This way, the backend can choose to filter private attrs.

Signed-off-by: Samuel Just <sam.just@inktank.com>
ECTransaction: support HashInfo checksums

Signed-off-by: Samuel Just <sam.just@inktank.com>
ECBackend: switch to HashInfo based checksums

Signed-off-by: Samuel Just <sam.just@inktank.com>