Merge XtraDB into 10.0

The storage/xtradb in the MariaDB 10.0 tree is currently mostly a copy
of the 5.5 code.

Before we can release 10.0, we need to merge XtraDB with the new InnoDB from
MySQL 5.6. XtraDB is the default InnoDB in MariaDB, and we need both the
user-visible features from XtraDB to avoid regressions in upgrades to 10.0, as
well as the general 5.6 improvements.

We do not need to merge all changes in XtraDB. Just the ones that could
reasonably be expected by users to remain. Some features of 5.5 XtraDB are
reimplemented in 5.6 InnoDB, so we can drop the XtraDB ones or possibly
implement config aliases etc. to remain compatible.

Probably most/all of the merge can be taken from Percona Server 5.6.

(I am logging this task mostly to ensure that we do not forget and end up
releasing MariaDB 10.0 beta/ga without XtraDB functionality, so user's setups
break in the upgrade due to unknown configuration options and such)