postgres: remove Pg-specific create_class

This version is still racy after several years.  More importantly,
it's missing the change in commit 5d01811ce47b888ca710985bf31ec45c4b7cadfa
which allows the default class to be overridden.
store: wrap create_class in a transaction to avoid races

Race conditions in create_class are unlikely to be a problem
in normal usage, but this will discourage code duplication
which can lead to maintainability issues.
domain removal also removes its default class

A default class may enter the class table if its settings
(e.g. mindevcount) are altered.  The queryworker does not
allow removing the default class, so the only way to remove
it is to remove it when the domain goes away.
tests: add "!want <count> <jobclass>" helper

This is used in several places, and will make code
easier to maintain going forward.
reaper: ensure worker can be stopped via "!want"

Now, all of our job classes may be controlled via "!want"