BUG #8728: Toast table much larger on replica

(apologies for lack of proper threading, but I never got an e-mail from my
form submission--checked spam folder and everything)

I took another look at the cluster, and the extra extents in question all
look like this in hexdump:

00000000 00 00 00 00 00 00 00 00 00 00 00 00 18 00 00 20
|............... |
00000010 00 20 04 20 00 00 00 00 00 00 00 00 00 00 00 00 |. .
............|
00000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
|................|
*
00002000 00 00 00 00 00 00 00 00 00 00 00 00 18 00 00 20
|............... |
00002010 00 20 04 20 00 00 00 00 00 00 00 00 00 00 00 00 |. .
............|
00002020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
|................|
*
00004000 00 00 00 00 00 00 00 00 00 00 00 00 18 00 00 20
|............... |

We're actually investigating a separate WAL-E issue that may have led to
missing files (possibly including visibility maps) when originally setting
up the base backup. I don't think that directly caused these extra files,
but I'm bringing it up since it may be relevant here.