MINOR: channel: Use dedicated functions to deal with STREAMER flags

MINOR: channel: Use dedicated functions to deal with STREAMER flags

For now, CF_STREAMER and CF_STREAMER_FAST flags are set in sc_conn_recv()
function. The logic is moved in dedicated functions.

First, channel_check_idletimer() function is now responsible to check the
channel's last read date against the idle timer value to be sure the
producer is still streaming data. Otherwise, it removes STREAMER flags.

Then, channel_check_xfer() function is responsible to check amount of data
transferred avec a receive, to eventually update STREAMER flags.

In sc_conn_recv(), we now use these functions.