genie_python: check set_pv and completion callbacks

set_pv_value() currently uses pend_io() when waiting for a completion callback. I think it should just need a flush_io() after the initial write and then just wait for the event to be signalled, but the code comments suggest otherwise. My only concern with pend_io() is that it also waits for outstanding get and pv search requests to finish, I don't know if this means that in a multi-threaded application your put + callback may get slightly delayed returning if a get or search happened to be being conducted by another thread at the same time.

* we need to write some tests that set pv and check values being progressed
* The confirm flush_io() behaviour