masterd: use AsyncTerminatedMessageStream for luxi

Each luxi connection now creates an asyncore MasterClientHandler (which
is an AsyncTerminatedMessageStream subclass, sending each message to a
client worker). This makes it harder to DOS the master daemon by just
creating luxi connections, as each of them will use memory and file
descriptors, but not a dedicated thread.

Each connection will only handle one message at a time.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>