ReplicatedPG,ReplicatedBackend: move over recovery message handlers

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGBackend: add is_primary, clear_state, dump_recovery_info

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG,ReplicatedBackend: move the push/pull message handling

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