For several commands (e.g. SET NAMES, SET character_set,
USE database, SET SQL_MODE) the client receives an ok packet
without information about server status changes.
e.g. After SET NAMES big5 the server assumes that the client
sends big5 encoded data, while the client character set is
still latin1.

This patch extends the OK packet to allow server to send
more information.

List of server changes transmitted:
- Session system variables
- current database (schema)
- info is sent as a lenenc string when
  CLIENT_SERVER_STATE_CHANGE is supported. Else the previous
  format is followed.

A resource manager is also introduced to support session
level char pointer system variables.