Fix: data pending race

There is a data pending race involving late population of the streams in
the stream hash table, and applying flush on streams that are not yet
globally visible.

This is caused by the fact that streams are added to the hash table only
when received by the data-handling consumer thread.

This results in data_pending() incorrectly returning that there is no
data pending in some cases.

This has been discovered by adding 1s delay in read subbuffer function
for testing.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>