Single Mapped Buffer qa tests fail occasionally

[mormj]

Have noticed 2 failure modes:

1. When logging is turned on, the flowgraph will occasionally hang
2. When logging is off, produces some number of samples less than the expected number

[mormj]

May have solved this with 2682c78

A lot of the test cases were failing intermittently because there were less samples in the sink than were transmitted
The better?? flushing logic should help this