MINOR: global: Add an option to disable the data fast-forward

MINOR: global: Add an option to disable the data fast-forward

The new global option "tune.fast-forward" can be set to "off" to disable the
data fast-forward. It is an debug option, thus it is internally marked as
experimental. The directive "expose-experimental-directives" must be set
first to use this one. By default, the data fast-forward is enable.

It could be usefull to force to wake the stream up when data are
received. To be sure, evreything works fine in this case. The data
fast-forward is an optim. It must work without it. But some code may rely on
the fact the stream will not be woken up. With this option, it is possible
to spot some hidden bugs.