Implement MasterSlave::get_slaves() to get cxns from a DSN table. Add comments explaining use_repl_db().

t/lib/samples/MasterSlave/dsn_table.sql

@@ -0,0 +1,12 @@
+DROP DATABASE IF EXISTS dsn_t;
+CREATE DATABASE dsn_t;
+USE dsn_t;
+
+CREATE TABLE dsns (
+  id int auto_increment primary key,
+  parent_id int default null,
+  dsn varchar(255) not null
+);
+
+INSERT INTO dsns VALUES
+  (null, null, 'h=127.1,P=12346,u=msandbox,p=msandbox');