BUG/MINOR: filters: fix silent compression disable with legacy "filter compression...

BUG/MINOR: filters: fix silent compression disable with legacy "filter compression" first

The legacy "filter compression" declaration stored a pointer on the proxy
compression options in the filter configuration at parsing time. When the
"compression" directives were placed after the filter declaration, the
options were not allocated yet, so the filter kept a NULL pointer and the
compression was silently disabled at runtime.

The compression options are now allocated when the filter is declared, the
"compression" directives parsed later reuse the same structure, in both
declaration orders.

This bug was introduced by the commit d759f474 ("MINOR: flt-http-comp: Use
compression options as filter config"), which started to store the pointer
on the proxy compression options in the filter configuration and to check
it at runtime, while it was previously always NULL and the options were
resolved at runtime through the frontend and backend proxies. There is no
need to backport this fix, it is a 3.5-specific issue.