registry: allow plugin and feature filter funcs to call registry API

registry: allow plugin and feature filter funcs to call registry API

Don't keep the registry locked whilst iterating over the plugins
or features with a filter function. This would deadlock if the
callback tried to access the registry from the function. Instead,
make a copy of the feature/plugin list and then filter it without
holding the registry lock. This is still considerably faster than
the alternative which would be to use a GstIterator.

https://bugzilla.gnome.org/show_bug.cgi?id=756738