ReplicatedPG: move control over do_request into ReplicatedPG

We'll want to conditionally dispatch into the pgbackend handler,
which can only be done from ReplicatedPG.

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd/: add initial PGBackend recovery interfaces and stubs

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