SET GLOBAL innodb_change_buffering has no effect

When type of the settable global variable innodb_change_buffering was changed from string to ENUM, the "shadow" variable ibuf_use stopped being updated as a result of SET GLOBAL innodb_change_buffering. Only on InnoDB startup, the parameter innodb_change_buffering would take effect.

We should remove the redundant variable ibuf_use and map the parameter to a single global variable.