Fix recursive script no-cacheable flushing

[xintenseapple]

Summary:

• Track request-local script evaluation depth so nested script evaluation cannot recursively invalidate no-cacheable variables consumed by an outer script pass.

Verification:

• git diff --check; ./auto/configure --without-http_rewrite_module; make -f objs/Makefile -j2

Fixes #27

[mdounin]

Suggested change looks wrong to me. In particular:

• It fails to address the test case provided in the issue comments (here, with volatile map and set).
• It breaks recursive flushing when it is needed. E.g., when a volatile map is derived from non-cacheable variables, such as in the following configuration:

    map $args $map_flush {
        volatile;
        default wrong;
        secret  good;
    }
    ...
    server {
        location /map_flush {
            set $args "wrong";
            set $temp "$map_flush";
            set $args "secret";
            set $temp "$temp:$map_flush";
            return 200 "X${temp}X";
        }

Overall, it looks like there is no easy solution for this problem. As a first step to address this and other issues in script evaluation, I've implemented a patch series which introduces script buffer overrun protection, so such issues, if any, won't cause buffer overflows, but will result in request processing errors. See issue comments for more details.