Easy way to switch default connection.

Hi.
## supply a Easy way to switch default connection.


As we know all the DB kind obeject use \Database_Connection::instance() to connection.
for example:
  A,B modules Use master,slave connection.
A switch connection to master just:
```php
\Database_Connection::$default = "master"
```
And B switch connection to slave just:
```php
\Database_Connection::$default = "slave"
```
end than modules work out must give back it.
```php
\Database_Connection::$default = "default"
```

## Bad thing is.
User ma be very sure hie use which one  connection.