Get rid of slru.c's hardwired insistence on a fixed number of slots per

SLRU area.  The number of slots is still a compile-time constant (someday
we might want to change that), but at least it's a different constant for
each SLRU area.  Increase number of subtrans buffers to 32 based on
experimentation with a heavily subtrans-bashing test case, and increase
number of multixact member buffers to 16, since it's obviously silly for
it not to be at least twice the number of multixact offset buffers.