osd/: add PGBackend interfaces and stubs

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD,ReplicatedPG: let PGBackend handle the temp collection

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: pass on dump_recovery_info to PGBackend

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG,ReplicatedPG: handle do_request in ReplicatedPG,PGBackend

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: extract PGBackend::Listener recovery callbacks

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd/: allow dout for ReplicatedBackend methods to work in ReplicatedPG.cc

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG/Backend: shuffle sub_op_push over to ReplicatedBackend

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