[MINOR] frontend: add a new analyser to parse a proxied connection

[MINOR] frontend: add a new analyser to parse a proxied connection

The introduction of a new PROXY protocol for proxied connections requires
an early analyser to decode the incoming connection and set the session
flags accordingly.

Some more work is needed, among which setting a flag on the session to
indicate it's proxied, and copying the original parameters for later
comparisons with new ACLs (eg: real_src, ...).