MINOR: http_fetch: Add smp_prefetch_htx

MINOR: http_fetch: Add smp_prefetch_htx

It does the same than smp_prefetch_http but for HTX messages. It can be called
from an HTTP proxy or a TCP proxy. For HTTP proxies, the parsing is handled by
the mux, so it does nothing but wait. For TCP proxies, it tries to parse an HTTP
message and to convert it in a temporary HTX message. Sample fetches will use
this temporary variable to do their job.