MEDIUM: hlua_fcn: dynamic server iteration and indexing

MEDIUM: hlua_fcn: dynamic server iteration and indexing

This patch proposes to enumerate servers using internal HAProxy list.
Also, remove the flag SRV_F_NON_PURGEABLE which makes the server non
purgeable each time Lua uses the server.

Removing reg-tests/cli_delete_server_lua.vtc since this test is no
longer relevant (we don't set the SRV_F_NON_PURGEABLE flag anymore)
and we already have a more generic test:
  reg-tests/server/cli_delete_server.vtc

Co-authored-by: Aurelien DARRAGON <adarragon@haproxy.com>