rb://1546
approved by: Sunny Bains and Marko Makela

Our dealing of buf_page_t::access_time flag is inaccurate.
* If LRU eviction has not started we don't set the access_time
* If LRU eviction is started we set it only if the block is not
'too old'.
* Not a correctness issue but we hold buf_pool::mutex when
setting the flag

This patch fixes this by:
* Setting flag unconditionally whenever the first page access happens
* Use buf_page_t mutex to protect write to the flag