Improve performance of get_tags_by_area_in_contexts SQL query

Tim noticed while reviewing MDL-61135 that the query for get_tags_by_area_in_contexts will not be very performant.

It can easily be re-written to perform better.

Andrew suggested something along the lines of:

select distinct t.* from mdl_tag t inner join mdl_tag_instance ti on t.id = ti.tagid where component = 'abc' and itemtype = 'def' and contextid in (1,2,3,4,5);