MEDIUM: backend: move all LB algo parameters into an union

MEDIUM: backend: move all LB algo parameters into an union

Since all of them are exclusive, let's move them to an union instead
of eating memory with the sum of all of them. We're using a transparent
union to limit the code changes.

Doing so reduces the struct lbprm from 392 bytes to 372, and thanks
to these changes, the struct proxy is now down to 6480 bytes vs 6624
before the changes (144 bytes saved per proxy).