MEDIUM: hlua: Dynamic list of frontend/backend in Lua

MEDIUM: hlua: Dynamic list of frontend/backend in Lua

When HAproxy is loaded with a lot of frontends/backends (tested with 300k),
it is slow to start and it uses a lot of memory just for indexing backends
in the lua tables.

This patch uses the internal frontend/backend index of HAProxy in place of
lua table.

HAProxy startup is now quicker as each frontend/backend object is created
on demand and not at init.
This has to come with some cost: the execution of Lua will be a little bit
slower.