bufmgr: Use atomic sub for unpinning buffers

bufmgr: Use atomic sub for unpinning buffers

The prior commit made it legal to modify BufferDesc.state while the buffer
header spinlock is held. This allows us to replace the CAS loop
inUnpinBufferNoOwner() with an atomic sub. This improves scalability
significantly. See the prior commits for more background.

Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>
Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff