libwebsockets

lws_bind_protocol

When using http/1.1+ keepalive and mounts, the relationship between
a connection and a protocol becomes dynamic.  The same connection might
visit different bits of the url space served by different mounts using
different protocols.

This patch ensures protocols can cleanly manage their per-connection
allocations by using the following callbacks when the protocol changes

 LWS_CALLBACK_HTTP_BIND_PROTOCOL
 LWS_CALLBACK_HTTP_DROP_PROTOCOL

For example if the pss wants to malloc stuff at runtime, it should do it
in LWS_CALLBACK_HTTP_BIND_PROTOCOL or later, and clean it up in
...DROP_PROTOCOL.

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