pt-online-schema-chnage: Added plugin feature 'override_slavelag_check' which allows modules to create their own ReplicaLagWaiter callback.

I fixed the current tests.

bin/pt-online-schema-change

@@ -8120,6 +8120,12 @@ sub main {
          return $ms->get_slave_lag($dbh);
       };
 
+      # The plugin is able to override the slavelag check so tools like pt-heartbeat
+      # or other replicators (Tungsten...) can be used to measure replication lag
+      if ( $plugin && $plugin->can('override_slavelag_check') ) {
+         $get_lag = $plugin->override_slavelag_check();
+      }
+      
       $replica_lag = new ReplicaLagWaiter(
          slaves   => $slave_lag_cxns,
          max_lag  => $o->get('max-lag'),
@@ -11345,6 +11351,7 @@ These hooks, in this order, are called if defined:
    after_drop_old_table
    before_drop_triggers
    before_exit
+   override_slavelag_check
 
 Each hook is passed different arguments.  To see which arguments are passed
 to a hook, search for the hook's name in the tool's source code, like:

t/pt-online-schema-change/plugin.t

@@ -56,6 +56,7 @@ my @called = $output =~ m/^PLUGIN \S+$/gm;
 is_deeply(
    \@called,
    [
+      'PLUGIN override_slavelag_check',
       'PLUGIN init',
       'PLUGIN before_create_new_table',
       'PLUGIN after_create_new_table',
@@ -70,7 +71,7 @@ is_deeply(
       'PLUGIN before_drop_old_table',
       'PLUGIN after_drop_old_table',
       'PLUGIN before_drop_triggers',
-      'PLUGIN before_exit'
+      'PLUGIN before_exit',
    ],
    "Called all plugins on basic run"
 ) or diag(Dumper(\@called));

t/pt-online-schema-change/samples/plugins/all_hooks.pm

@@ -96,4 +96,11 @@ sub before_exit {
    print "PLUGIN before_exit\n";
 }
 
+sub override_slavelag_check {
+   my ($self, %args) = @_;
+   print "PLUGIN override_slavelag_check\n";
+
+   return sub { return 0; };
+}
+
 1;