Add innodb_adaptive_flushing_method and innodb_adaptive_checkpoint tunables

The following tuning is useful for optimizing DB workloads on flash storage, Fusion IO devices, SSD, etc.  They are useful for people using Percona or MariaDB.

innodb_adaptive_checkpoint on 5.1 is the same as innodb_adaptive_flushing_method on 5.5:

http://www.percona.com/doc/percona-server/5.5/scalability/innodb_io_55.html#innodb_adaptive_flushing_method

http://www.percona.com/doc/percona-server/5.1/scalability/innodb_io.html?id=percona-server:features:innodb_io_51&redirect=1#innodb_adaptive_checkpoint