osd: Implement multiple backfill target handling

Fixes: #5858

Signed-off-by: David Zafman <david.zafman@inktank.com>
Fix do_op() logic (will squash)
Fixes based on some of the code review comments
Fix for targ/src multiop
Fix all_peer_done() (renamed) so we terminate backfill correctly
Recovery reservations need to include backfill peers

Caused by 86e4fd4f7a9848511e1bf9ae433fcd3a35a0818f

Signed-off-by: David Zafman <david.zafman@inktank.com>
More code review changes
For testing override compatibility

This commit must NOT be merged to master