buffer: save pointers to map, fixes #6

[brodo]

Hi!
the problem I encountered in #6 was that the pointer to the callback might be GCed between the callback registration and the call of it. So now I'm storing the pointer in a hash table and then delete if when It was used. This fixes the issue for me. The same problem should exist for the error callback, but I could not find where that is handled, so I did not do anything with that for now.

[oliverbestmann]

While this might work, it just mirrors what cgo.Handle is already doing. The real issue here is passing a pointer to a cgo.Handle to C code is invalid. You correctly realized, that the memory the pointer points to can be garbage collected before the callback is invoked.
The thing is: a cgo.Handle is already pointer sized (it is actually a uintptr), so we can just pass the cgo.Handle itself to the C code. The value referenced by the handle is already stored in a sync.Map when you call cgo.Handle and will still be available in the callback.