MEDIUM: cache: Add support for endp-to-endp fast-forwarding

MEDIUM: cache: Add support for endp-to-endp fast-forwarding

It is now possible to directly forward data to the opposite side from the
cache applet. To do so, dedicated functions were added to fast-forward the
payload part of the cached objects. Of course headers and trailers are still
sent via the channel's buffer, using the HTX.

When an object is delivered from the cache, once the applet reaches the
HTX_CACHE_DATA state, it declares it can fast-forward data. From this point,
all data are directly transferred to the oppposite side.