Modules: added nocache flag for js_set variables.

Modules: added nocache flag for js_set variables.

This commit adds support for an additional `nocache` flag in `js_set`
directives. If set, the resulting nginx variable will have no_cacheable set
to 1. This enables us to dynamically recompute a variable if the context
changed (for example, in case of an internal redirection).

In case of multiple calls in a location, users should cache the result in a
rewrite variable: `set $cached_variable $js_variable;`