This patch introduces heuristics based flushing rate of dirty pages to
avoid IO bursts at checkpoint.

1) log_capacity / log_generated per second gives us number of seconds
in which ALL dirty pages need to be flushed. Based on this rough
assumption we can say that
n_dirty_pages / (log_capacity / log_generation_rate) = desired_flush_rate

2) We use weighted averages (hard coded to 20 seconds) of
log_generation_rate to avoid resonance.

3) From the desired_flush_rate we subtract the number of pages that have
been flushed due to LRU flushing. That gives us pages that we should
flush as part of flush_list cleanup. And that is the number (capped by
maximum io_capacity) that we try to flush from the master thread.

Knobs:
======

innodb_adaptive_flushing: boolean, global, dynamic, default TRUE.
Since this heuristic is very experimental and has the potential to
dramatically change the IO pattern I think it is a good idea to leave a
knob to turn it off.

Approved by: Heikki