Allow PlainTextAuthProvider to change its credentials at runtime

Final decision:

Add setUsername and setPassword to PlainTextAuthProvider directly. This is trivial and backward-compatible, so there is no need to introduce a separate class.

Initial report:
PlainTextAuthProvider allows user to set username and password only at the construction time. In a typical production environment, the client occasionally wants to change the user-name/password at run time with out taking a bounce. Users can easily write their own version of PlainTextAuthProvider but it will be good for driver to provide a DynamicPlainTextAuthProvider for all users.