Implement the Percona variable slow_query_log_always_write_time

Documentation from Percona:
This variable can be used to specify the query execution time after which the query will be written to the slow query log. It can be used to specify an additional execution time threshold for the slow query log, that, when exceeded, will cause a query to be logged unconditionally, that is, log_slow_rate_limit will not apply to it."

The Percona implementation shows that slow_query_log_always_write_time only comes into effect for queries longer than long_query_time.