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>