Sequence of events:
1) HTTP request is being handled in worker thread by calling the handler
func() from within websocket_process().
2) Handler func generates output, queues it up, and requests a
`writeable` callback from within websocket_write_raw().
3) Main LWS thread triggers writeable callback and calls
websocket_dequeue().
4) Output is given to LWS still within the main LWS thread, and finally
lws_http_transaction_completed() is called to release the connection
and ready it for the next HTTP connection.
5) LWS internally cleans up the connection and frees the user context
(our `wc` struct).
6) The worker thread wakes up and continues to use the now invalid `wc`
in order to clean up after it has done its job. Boom.
The solution is to handle the `drop protocol` callback, which is
triggered by LWS in the main LWS thread in step 4 from within
lws_http_transaction_completed(). We call our own connection cleanup
function websocket_conn_cleanup() which blocks until all jobs are
removed from `wc` (step 6) and only then continue, allowing LWS to
safely free the struct.
Change-Id: I596a98e9b552a96aef259f4523f16fa63c287ef4
Richard Fuchs
9653546f7c