Worker-local BufferProvider implementation

[Ichthyostega]

As a Lumiera developer,
I need a thread-local flavour of both BufferProvider back-ends,
so that I can configure a BufferProvider instance capable to operate in a massive parallel environment.

envisaged solution

• with #1425 an adapter was developed to operate the base implementation in a thread-local setup
• for the BufferStore implementation, it seems fairly straight forward to create a simple implementation that uses a small local allocation pool, while resolving any memory discrepancy directly with heap allocations / deallocations
• the BufferStage interface is a bit more challenging, since it embeds a hierarchical BufferMetadata table. For a preliminary implementation however, it seems sufficient to add a function for exchange of medatata chains. This could then be used with simple Mutex-based locking in those (exceptional) cases where a thread-local slice is confronted with a buffer allocation request that refers to a buffer type not known locally.

[Ichthyostega]

Attach to parent #1428 (Engine Buffer Manager).

[Ichthyostega]

Attach to parent #1429 (control effort for global buffer allocator).