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>
5 files changed