sd-bus: add APIs to request/release names asynchronously

sd-bus: add APIs to request/release names asynchronously

They do the same thing as their synchronous counterparts, but only
enqueue the operation, thus removing synchronization points during
service initialization.

If the callback function is passed as NULL we'll fallback to generic
implementations of the reply handlers, that terminate the connection if
the requested name cannot be acquired, under the assumption that not
being able to acquire the name is a technical problem.