BUG/MAJOR: http-htx: Store new host in a chunk for scheme-based normalization

BUG/MAJOR: http-htx: Store new host in a chunk for scheme-based normalization

During the scheme based normalization, The original authority value is used
to replace every host headers. It is an issue, because when the HTX message
is modified the blocks may be reorganised to find free space. For instance a
defragmentation can be performed. So the address of the authority can
change. To perform such rewrite, the new value must be stored in a temporary
buffer. It is especially important because the start-line is also updated,
so the original authority could be moved, making it invalid.

Because of this bug, it is possible to mix HTX block values. There is no
overflow but the start-line can be crushed with data from the host header
value, making it invalid for the server.

So, to fix the issue, the new host header value is now the one in the trash
chunk used to rewrite the start line. But in that case, the trash chunk must
be allocated to be sure it remains valid when replacing all host headers
values.

This patch must be backported as far as 2.6.