Test asynchronous command processing

[Cito]

In version 5.2 we integrated support for asynchronous command processing which has been contributed on the mailing list by Patrick TJ McPhee a while ago (see #19).

However, this feature is still experimental and only rudimentary tested. We need much more testing.

• nowait parameter
• method poll(), set_non_blocking(), is_non_blocking().
• method send_query() and all ways of getting results afterwards
  (iterators, scalars, one, single, direct access etc.)