add separate server writeable and change naming to match rest of callbacks

add separate server writeable and change naming to match rest of callbacks

Notice that the naming is changed, the notification to a server that it can write to
the client is now called LWS_CALLBACK_SERVER_WRITEABLE, and the notification to a client
that it can write to a server is LWS_CALLBACK_CLIENT_WRITEABLE.

Signed-off-by: Andy Green <andy@warmcat.com>