sessiond: notification: introduce the notion of 'active' client

Since notification_clients are now accessed from multiple threads, it
is possible for a thread to access a client while it is being
"cleaned-up" following an error.

The 'active' communication flag allows a check to be performed before
any communication is attempted with a client. The communication is
considered 'active' once the handshake has been performed. It is
considered 'inactive' if a fatal protocol error occurs.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: If16f83ef12fe9a6ef597ae464867011811389177