Update database schema for queue_classic 3.0.

- Add columns created_at and locked_by.
- Add NOT NULL constraints.
- Add notify trigger.
Add dummy space to fix Vim syntax highlighting.
Add `unlock-deads` sub-command.

Also set the `locked_by` field, which is used by `unlock-deads`. See help
message and associated comment in the code for a description.
Let PostgreSQL know who we are (e.g. for `ps`).
Use LISTEN/NOTIFY for rapid worker wakeup.
Use normal PostgreSQL connection string in tests.

Also use strict bytestrings in some data types.
Merge branch 'queue_classic_3_0'