bufmgr: Make it easier to change number of buffer state bits

bufmgr: Make it easier to change number of buffer state bits

In an upcoming commit I'd like to change the number of bits for the usage
count (the current max is 5, fitting in three bits, but we reserve four
bits). Until now that required adjusting a bunch of magic constants, now the
constants are defined based on the number of bits reserved.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/lxzj26ga6ippdeunz6kuncectr5gfuugmm2ry22qu6hcx6oid6@lzx3sjsqhmt6
Discussion: https://postgr.es/m/riivolmg6uzfvpzfn6wjo3ghwt42rcec43ok6mv4oenfg654y7@x7dbposbskwd