make insert buffer more controlable

[Yasufumi Kinoshita]

* add parameter to limit insert buffer size
    current limit of the size is the half of the buffer pool.
    it may be too large when we use huge buffer pool (over several GB).

* add parameter to make the merging more active
    currently, we can control the activity by innodb_io_capacity.
    but, it may be still insufficient for high-end hardware.

[Evgeniy Stepchenko (evgeny-stepchenko)]

replaced by https://blueprints.launchpad.net/percona-xtradb/+spec/control-merging-insert-buffer and https://blueprints.launchpad.net/percona-xtradb/+spec/restrict-insert-buffer-size