libwebsockets

http: body: make sure to consume body before transaction complete

https://github.com/warmcat/libwebsockets/issues/1625

"dead bodies" that were sent but not processed by lws as server
will clog up and destroy transaction tracking if repeated POSTs
with keepalive are sent to nonexistant paths.

This patch introduces a DISCARD_BODY state that follows BODY
except the payload is not signalled to the protocol callback.
Calling transaction_completed() with pending body makes lws
enter DISCARD_BODY and retry transaction completed only after
the pending body is exhausted.