BUG/MEDIUM: hlua/filters: Make Channel.may_recv relative to the filter's data

BUG/MEDIUM: hlua/filters: Make Channel.may_recv relative to the filter's data

Channel.may_recv() tested the whole channel to know if more data may be
received, ignoring the data visible by the calling filter. But when several
Lua filters are chained on a channel and an earlier one holds back some data
(for instance to rewrite the body), the channel input may be closed while
the current filter still waits for data held upstream. In that case, the
function must return true, because the held data can still be exposed to
the filter, even if nothing will be received on the connection anymore.

Now, like HTTPMessage.eom(), HTTPMessage.may_recv() and
HTTPMessage.closed(), the result is relative to the data visible by the
current filter: the function always returns true if an earlier filter
holds back data.

This patch should be backported to all supported versions.