public class ResolveExternals implements Queueable {
  String[] externals;
  public ResolveExternals(String[] values) {
    externals = values;
  }
  public void execute(QueueableContext context) {
    Event[] events = [SELECT ... FROM Event WHERE External_Id__c = :externals FOR UPDATE];
    Contact[] contacts = [SELECT ... FROM Contact WHERE External_Id__c = :externals FOR UPDATE];
    // match results
  }
}

String[] externals = new String[0];
for(... record: Trigger.new) {
  externals.add(record.External_Id__c);
}
System.enqueueJob(new ResolveExternals(externals));