The optional target queue is useful for synchronising other work with
respect to database operations. Provided the target queue is also a
serial queue, work submitted to the private connection queue and work
submitted directly to the target queue will still be executed in serial.

The queue context is now set on the target queue if present. The
connection queue inherits values set on the target queue, so the call to
`DispatchQueue.getSpecific`, using `queueKey`, returns `queueContext` as
expected. For this logic to still be thread-safe, the target queue must
be a serial queue. `queueKey` is now an instance variable so that every
connection gets a unique key, ensuring that connections can't write over
one another's context variables.

The `attributes:` parameter is also removed from the call to initialise
the database queue, as `[]` (i.e., serial) is already the default.