reaper: add "queue_rate_for_reaper" server setting

This controls the number of FIDs the reaper can inject into the
replication queue for each dead device, per wakeup.

This defaults to 1000, the same value its had since (at least) 2006.

lib/MogileFS/Worker/Reaper.pm

@@ -65,9 +65,10 @@ sub work {
             my $sto = Mgd::get_store();
             my $lock = "mgfs:reaper";
             my $lock_timeout = $self->watchdog_timeout / 4;
+            my $limit = MogileFS::Config->server_setting_cached('queue_rate_for_reaper') || 1000;
 
             if ($sto->get_lock($lock, $lock_timeout)) {
-                my @fids = $dev->fid_list(limit => 1000);
+                my @fids = $dev->fid_list(limit => $limit);
                 if (@fids) {
                     $self->still_alive;
                     foreach my $fid (@fids) {

t/70-reaper.t

@@ -51,6 +51,7 @@ ok($tmptrack->mogadm("device", "add", "hostA", 2), "created dev2 on hostA");
 
 ok($tmptrack->mogadm("domain", "add", "testdom"), "created test domain");
 ok($tmptrack->mogadm("class", "add", "testdom", "2copies", "--mindevcount=2"), "created 2copies class in testdom");
+ok($tmptrack->mogadm("settings", "set", "queue_rate_for_reaper", 123), "set queue_rate_for_reaper");
 
 # create one sample file with 2 copies
 my $fh = $mogc->new_file("file1", "2copies");