Fix #384: change handling of SmileBufferRecycler to be thread-safe

[cowtowncoder]

Simplify SmileBufferRecycler by using AtomicReference to guard access to buffers. This didn't use to be problematic before async handling (when there was typically 1-to-1 matching between threads and Parser/Generator instances).
However, this changed for both:

1. Async Smile parser
2. Non-blocking frameworks where access to input is not handled with BIO (and as a result there's no thread-to-parser/generator binding)

and so there's no guarantee of atomicity for reuse by recycler.

[cowtowncoder]

@simondaudin I think this should fix it; will merge but would appreciate if you could verify on your end. Targeting 2.15 branch for eventual 2.15.3 release (and 2.16.0).

[simondaudin]

@cowtowncoder Thank you for the fix, that was fast !
I tested with:

• The test I has given in the issue -> OK
• A test I had with some of our application logic -> also OK
  For my point of view, it's fixed.