AlreadyReading flag not used correctly

AlreadyReading flag not used correctly

 - alreadyReading field is set correctly in channelRead() to signal that
   reading occurs and that any tries to flush are not needed, as a flush
   is always scheduled after the reading is done
 - alreadyReading flag was not set back to false, which
   might cause unnecessary checks for flush being scheduled

Change-Id: Icafdf282390340b176cbe4d9702c84558af8b412
Signed-off-by: Michal Polkorab <michal.polkorab@pantheon.sk>
(cherry picked from commit 1de9685954275657b096f2e4f325f247028cb7e8)