MDEV-13670/MDEV-14550 Error log flood :  "InnoDB: page_cleaner: 1000ms intended loop took N ms. The settings might not be optimal."

Silence the error log output that was introduced in MySQL 5.7
(MariaDB 10.2.2) if log_warnings=2 or less.

We should still figure out what these messages really indicate
and how to solve the problems.

pc_sleep_if_needed(): Add a parameter for the current time,
so that there will be fewer successive calls to ut_time_ms()
with no I/O between them.

buf_flush_page_cleaner_coordinator(): Exit the first loop
whenever shutdown has been requested. At the start of the loop,
call ut_time_ms() only once. Do not display the message if
log_warnings=2 or less.