Callback for H5Pset_append_flush() is never invoked by library

**Describe the bug**
The function pointer gets stored, but is never called.

**Expected behavior**
Call it according to the spec for H5Pset_append_flush()

**Platform (please complete the following information)**
All

**Additional context**
It is semi-tested in test/swmr.c: the set/get of the property is checked, but there's no actual validation that the callback is invoked.