Improve cold-start performance of RWS

And suppress lots of warnings.

By loading subchanges in order we will only append to lists, which is
way more efficient than resetting the per-user/per-task history each
time.
Fix bug in have_changed to output_only task

When a contest contains output_only task, cmsReimporter throws an error
such that sol is not found.

Fix it by adding if-statement that checks task type.
To achieve it, initialize conf.