[MINOR] backend: separate declarations of LB algos from their lookup method

[MINOR] backend: separate declarations of LB algos from their lookup method

LB algo macros were composed of the LB algo by itself without any indication
of the method to use to look up a server (the lb function itself). This
method was implied by the LB algo, which was not very convenient to add
more algorithms. Now we have several fields in the LB macros, some to
describe what to look for in the requests, some to describe how to transform
that (kind of algo) and some to describe what lookup function to use.

The next patch will make it possible to factor out some code for all algos
which rely on a map.