PG: retry GetLog() each time we get a notify in Incomplete

If for some reason there are no up OSDs in the history which
happen to have usable copies of the pg, it's possible that
there is a usable copy elsewhere on the cluster which will
become known to the primary if it waits.

Fixes: #6909
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
PG: don't query unfound on empty pgs

When the replica responds, it responds with a notify
rather than a log, which the primary then ignores since
it is already in the peer_info map.  Rather than fix that
we'll simply not send queries to peers we already know to
have no unfound objects.

Fixes: #6910
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>