widen vacuum buffer counters

We recently noticed that vacuum buffer counters wraparound in extreme
cases, with ridiculous results.  Example:

2020-01-06 16:38:38.010 EST [45625-1] app= LOG:  automatic vacuum of table "somtab.sf.foobar": index scans: 17
        pages: 0 removed, 207650641 remain, 0 skipped due to pins, 13419403 skipped frozen
        tuples: 141265419 removed, 3186614627 remain, 87783760 are dead but not yet removable
        buffer usage: -2022059267 hits, -17141881 misses, 1252507767 dirtied
        avg read rate: -0.043 MB/s, avg write rate: 3.146 MB/s
        system usage: CPU 107819.92s/2932957.75u sec elapsed 3110498.10 sec

That's to be expected, as tables exist that are large enough for 4 billion
buffer accesses to be a possibility.  Let's widen the counters, as in the
attached patch.

I propose to backpatch this.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services