bufmgr: Turn BUFFER_LOCK_* into an enum

bufmgr: Turn BUFFER_LOCK_* into an enum

It seems cleaner to use an enum to tie the different values together. It also
helps to have a more descriptive type in the argument to various functions.

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