diag: diagfwd_socket: Fix DEL_CLIENT notification

The QMI handle used to register server lookups will never receive
DEL_CLIENT notifications because itself is a client. Only servers will
receive the DEL_CLIENT notification. Remove the DEL_CLIENT handler from
the QMI handle.

The sockets that act as servers should parse control packets to see if
the client they are connected to ever disconnects. Add handling for the
bye and del_client control packets.

Since the SSR Framework is no longer used to receive side channel state
notifications, the socket that acts as the server should always remain
open. During client disconnect cleanup, close the channel in the diag
layers but keep the socket open so new data packets can reopen the diag
channel.

Change-Id: I64d1793a1335ee4b50ad264f78d91a43ccd5279a
Signed-off-by: Chris Lew <clew@codeaurora.org>