Draft: Fix threaded device allocation

We are starting to introduce threads in the VTK-m control environment so that we can schedule multiple things at once. Once issue we ran into is that some methods to allocate data on devices are not thread safe. To fix this, we make allocating memory for a Buffer thread safe. This could potentially slow things down waiting for mutexes, but you are hopefully not spending a lot of time allocating/deallocating memory anyway.