bufmgr: Separate keys for private refcount infrastructure

bufmgr: Separate keys for private refcount infrastructure

This makes lookups faster, due to allowing auto-vectorized lookups. It is also
beneficial for an upcoming patch, independent of auto-vectorization, as the
upcoming patch wants to track more information for each pinned buffer, making
the existing loop, iterating over an array of PrivateRefCountEntry, more
expensive due to increasing its size.

Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/fvfmkr5kk4nyex56ejgxj3uzi63isfxovp2biecb4bspbjrze7@az2pljabhnff