failing test: async results do not decrement buffer counter

This test demonstrates that all results delivered by gen_fsm:send_event
are not counted against the buffer limit. This means that MR pipes using
FSM sink type are effectively using a buffer 10x the size they think
they are, because gen_fsm:*sync_*send_event is only used once every 10
results.
count async MR results against the sink buffer size cap

This fixes the problem demonstrated by the test in the previous commit.
Merge pull request #529 from branch 'bwf-async-sink-buffer'