Recheck the list of slaves in the table specified with DSN

This is to add/remove monitored machines while the OSC runs.

This contains work by:
- Daniël van Eeden <daniel.vaneeden@booking.com>
- Mikhail Izioumtchenko <mikhail.izioumtchenko@booking.com>

lib/ReplicaLagWaiter.pm

@@ -80,6 +80,26 @@ sub wait {
    my $worst;  # most lagging slave
    my $pr_callback;
    my $pr_first_report;
+
+   ### refresh list of slaves. In: self passed to wait()
+   ### Returns: new slave list
+   my $pr_refresh_slave_list = sub {
+      my ($self) = @_;
+      my ($slaves, $refresher) = ($self->{slaves}, $self->{get_slaves_cb});
+      return if ( not defined $refresher );
+      my $before = join ' ', sort map {$_->name()} @$slaves;
+      $slaves = $refresher->();
+      my $after = join ' ', sort map {$_->name()} @$slaves;
+      if ($before ne $after) {
+         $self->{slaves} = $slaves;
+         printf "Slave set to watch has changed\n  Was: %s\n  Now: %s\n",
+            $before, $after;
+      }
+      return($self->{slaves});
+   };
+
+   $slaves = $pr_refresh_slave_list->($self);
+
    if ( $pr ) {
       # If you use the default Progress report callback, you'll need to
       # to add Transformers.pm to this tool.
@@ -113,6 +133,15 @@ sub wait {
    my @lagged_slaves = map { {cxn=>$_, lag=>undef} } @$slaves;  
    while ( $oktorun->() && @lagged_slaves ) {
       PTDEBUG && _d('Checking slave lag');
+
+      ### while we were waiting our list of slaves may have changed...
+      $slaves = $pr_refresh_slave_list->($self);
+      my $watched = 0;
+      @lagged_slaves = grep {
+         my $slave_name = $_->{cxn}->name();
+         grep {$slave_name eq $_->name()} @{$slaves // []}
+                            } @lagged_slaves;
+
       for my $i ( 0..$#lagged_slaves ) {
          my $lag = $get_lag->($lagged_slaves[$i]->{cxn});
          PTDEBUG && _d($lagged_slaves[$i]->{cxn}->name(),