Based on contribution from Google Inc.
This patch introduces a new parameter innodb_io_capacity to control the
rate at which master threads performs various tasks. The default value
is 200 and higher values imply more aggressive flushing and ibuf merges
from within the master thread.
This patch also changes the ibuf merge from synchronous to asynchronous.
Another minor change is not to force the master thread to wait for a
log flush to complete every second.

Approved by: Heikki