BUG/MINOR: peers: Use right channel flag to consider the peer as connected

BUG/MINOR: peers: Use right channel flag to consider the peer as connected

When a peer open a new connection to another peer, it is considered as
connected when the hello message is sent. To do so, the peer applet was
relying on CF_WRITE_PARTIAL channel flag. However it is not the right flag
to use. This one is a transient flag. Depending on the scheduling, this flag
may be removed by the stream before the peer has a chance to see
it. Instead, CF_WROTE_DATA flag must be checked.

This patch is related to the issue #1799. It must be backported as far as
2.0.