BUG/MINOR: hlua: fix reference leak in core.register_task()

BUG/MINOR: hlua: fix reference leak in core.register_task()

In core.register_task(): we take a reference to the function passed as
argument in order to push it in the new coroutine substack.
However, once pushed in the substack: the reference is not useful
anymore and should be cleared.
Currently, this is not the case in hlua_register_task().

Explicitly dropping the reference once the function is pushed to the
coroutine's stack to prevent any reference leak (which could contribute
to resource shortage)

This may be backported to every stable versions.
Please note that this commit depends on "MINOR: hlua: add simple hlua reference handling API"