MEDIUM: http-ana: Use a large buffer if necessary when waiting for body

MEDIUM: http-ana: Use a large buffer if necessary when waiting for body

Thanks to previous patches, it is now possible to allocate a large buffer to
store the message payload in the context of the "wait-for-body" action. To
do so, "use-large-buffer" option must be set.

It means now it is no longer necessary to increase the regular buffer size
to be able to get message payloads of some requests or responses.