Kill the serialization of $wp_filter in get_terms()

[nacin]

We use this as part of a persistent cache key:

serialize($GLOBALS['wp_filter']['list_terms_exclusions'])`

See [8225].

This is not good for two reasons. First, well, it's a not fun to reach into our internal API like that. But worse, it's broken whenever an object method is used, because spl_object_hash() will be unique to that object, thus rendering the cache invalidated on the next pageload.

As an aside, we should probably have _wp_filter_build_unique_id() create a delimiter when dealing with a static class method — $function[0].$function[1] can conflict with a legitimate function name.

I'm not sure how this should be fixed.

That looks incredibly hacky. Besides, closures are not serializable (you get a fatal error).