MINOR: peers: Use a peer flag to block the applet waiting ack of the sync task

MINOR: peers: Use a peer flag to block the applet waiting ack of the sync task

Since recent fixes on peers, some changes on a peer must be acknowledged
by the sync task before letting the peer applet processing messages.
Blocking conditions was based on a combination of flags. It was
errorprone. So, this patch introduces PEER_F_WAIT_SYNCTASK_ACK peer flag for
this purpose. This flag is set by the peer when it must wait for an ack from
the sync task. This sync task, on its side, must remove it and wake the peer
up.