Moved the packet parsing and creation of commands(COM_STMT_EXECUTE,
COM_STMT_FETCH, COM_STMT_SEND_LONG_DATA, COM_STMT_PREPARE,
COM_STMT_CLOSE, COM_STMT_RESET) to the Protocol classes as this is
protocol-dependent, this way enabling PROTOCOL_PLUGINs to execute
prepared statements.

- added stack-like mechanism for switching between protocols
- added store_ps_status to the protocol interface for sending the
statement id and the metadata related to the newly created prepared
statement
- added send_parameters to the protocol interface for sending the out
parameters to the client
- added the flush method to the protocol interface to send the current
stream to the clients (used by the classic protocols).
- added mysql_stmt_precheck to validate the PS
- added a send_statement method to sql_prepare