osd/: add initial PGBackend recovery interfaces and stubs

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: move the temp collection into PGBackend

Other, future uses of the temp collection will need
to come up with their own temp collections.

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: fill in on_local_recovery

PGBackend::Listener::on_local_recovery will be called with the
transaction completing the local recovery of an object.  This should
leave the remaining code in submit_push_* and friends able to be
moved into ReplicatedBackend.

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedBackend: fill in on_local_recover_start

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