This is a massive speed increase in journal_get_users_done.

This is a massive speed increase in journal_get_users_done.

The fix requires UNION support in the database.  I've added a simple little
test to see if UNION queries work.

This change very much needs to be tested out by someone using MySQL 3.  If
it tests out there successfully, the supports_union code should be moved out
of this file and into datalib.  Ideally, someone will extend that test and
creatle get_records_union_sql, a function that would split up UNION queries
for databases that don't support them and do an array_merge (and proper
sorting, which is missing from my fix) on the result sets.