Engine Buffer Manager

[Ichthyostega]

As a Lumiera developer,
I need a common back-end to maintain the pooling allocations,
so that the worker-local Buffer Provider can delegate the basic allocation management

draft

• the BufferProvider for productive use is organised such that it can be operated in »Slices« per thread (#1425)
• notably the BufferStore implementation maintains a LocalMemPool in each thread
• however these local pools can not perform allocations themselves, since that would incur a global lock

Thus the implementation class LocalBufferStore is connected via asynchronous messaging to some central instance, where a given amount of buffers can be requested

[Ichthyostega]

Attach to parent #1387 (Memory handling scheme for render buffers).