R1:22c64c67ffca

Fix performance problem for large task queues

Summary:
Some time ago, we added `ORDER BY id ASC` to the worker `UPDATE ...` query, because someone reported that their MySQL read slaves were complaining about the query (I can't find the exact error message, but something to the effect of the rows the query affected not being deterministic). This seemed harmless since it should be the same as the query's implicit order (I guess?), but actually made the query dramatically slower for large numbers of rows.