Limit conn: removed deprecated "limit_zone" directive.

It's deprecated since 260d591cb6a3 (1.1.8).  The "limit_conn_zone" directive
should be used instead.
Limit conn: use complex value in limit_conn_zone (ticket #121).

One intentional side effect of this change is that key is allowed only
in the first position.  Previously, it was possible to specify the key
variable at any position, but that was never documented, and is contrary
to nginx configuration practice for positional parameters.
Limit conn: aligned field names in structures.

No functional changes.
Limit req: use complex value in limit_req_zone.

One intentional side effect of this change is that key is allowed only
in the first position.  Previously, it was possible to specify the key
variable at any position, but that was never documented, and is contrary
with nginx configuration practice for positional parameters.
Limit req: reduced number of parameters in the lookup function.

No functional changes.