osd: Backfill peers should not be included in the acting set

Create actingbackfill in choose_acting()
Use first backfill target as previously
Add asserts to catch inappropriate use of actingbackfill
Use is_acting() in proc_replica_info() because this is before actingbackfill set
Remove backfill_targets from stray_set to prevent purge_strays from removing collection
Can't check is_replica() anymore for backfill operations since a backfill isn't
a replica due to acting set change.

fixes: #5855

Signed-off-by: David Zafman <david.zafman@inktank.com>

src/osd/OSD.cc

@@ -6011,8 +6011,11 @@ void OSD::dispatch_context_transaction(PG::RecoveryCtx &ctx, PG *pg)
 bool OSD::compat_must_dispatch_immediately(PG *pg)
 {
   assert(pg->is_locked());
-  for (vector<int>::iterator i = pg->acting.begin();
-       i != pg->acting.end();
+  vector<int> *tmpacting = &pg->acting;
+  if (pg->actingbackfill.size() > 0)
+    tmpacting = &pg->actingbackfill;
+  for (vector<int>::iterator i = tmpacting->begin();
+       i != tmpacting->end();
        ++i) {
     if (*i == whoami)
       continue;