bufmgr: Change BufferDesc.state to be a 64-bit atomic

bufmgr: Change BufferDesc.state to be a 64-bit atomic

This is motivated by wanting to merge buffer content locks into
BufferDesc.state in a future commit, rather than having a separate lwlock (see
commit c75ebc657ff for more details). As this change is rather mechanical, it
seems to make sense to split it out into a separate commit, for easier review.

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