MEDIUM: applet: Handle channel's STREAMER flags on applets size

MEDIUM: applet: Handle channel's STREAMER flags on applets size

Till now, it was not possible to notify an producing applet is streaming
data. It means, it was not possible to set CF_STREAMER and CF_STREAMER_FLAGS
on the input channel of an applet streaming data.

While it is not a big deal for most of applets, it is interesting for the
cache. Because there are now dedicated functions to deal with these flags,
we can use them in task_run_applet() to set/unset these flags on the input
channel.

This patch relies on "MINOR: channel: Use dedicated functions to deal with
STREAMER flags".