…ueue#39)

This one's related to riverqueue#38. It turns out that while trying to mock a
context manager kind of works, it will do the wrong thing in edge cases
like when an exception is thrown from inside a `with` block, silently
swallowing it and causing a return that's completely wrong.

There may be some way to fix the mock to make it do the right thing, but
instead of getting fancier with these mocks that are already awful,
instead repair the problem by defining a plain class that implements
context manager and just use that.

* use new unique jobs implementation

This moves the library to use the new unique jobs implementation from
riverqueue/river#590 and migrates the sqlalchemy
driver to use a unified insertion path, allowing bulk inserts to use
unique jobs.

* sort args before hashing, support partial arg extraction

* work around sqlc nullable array value type issue

* documentation updates, changelog

* remove whitespace from unique key json component

…e#40)