ReplicatedBackend: factor out be_deep-scrub

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGBackend/ReplicatedBackend: move the backend agnostic code into PGBackend for scrub

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGBackend/ReplicatedBackend: factor out logical to ondisk size mapping

Signed-off-by: Samuel Just <sam.just@inktank.com>
ECBackend: add scrub support

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