Fix MVCC bug with prepared xact with subxacts on standby

We did not recover the subtransaction IDs of prepared transactions
when starting a hot standby from a shutdown checkpoint. As a result,
such subtransactions were considered as aborted, rather than
in-progress. That would lead to hint bits being set incorrectly, and
the subtransactions suddenly becoming visible to old snapshots when
the prepared transaction was committed.

To fix, update pg_subtrans with prepared transactions's subxids when
starting hot standby from a shutdown checkpoint. The snapshots taken
from that state need to be marked as "suboverflowed", so that we also
check the pg_subtrans.

Discussion: https://www.postgresql.org/message-id/6b852e98-2d49-4ca1-9e95-db419a2696e0%40iki.fi

NEON: cherry-picked from the upstream thread ahead of time, to unblock
#7288. I expect this to be
committed to upstream in the next few days, superseding this. NOTE: I
did not include the new regression test on v15 and v14 branches,
because the test would need some adapting, and we don't run the perl
tests on Neon anyway.

vendor/revisions.json

@@ -1,5 +1,5 @@
 {
-  "v16": ["16.3", "9837db157837fcf43ef7348be0017d3a2238cd27"],
-  "v15": ["15.7", "e22098d86d6c40276b6bd75c29133a33fb283ab6"],
-  "v14": ["14.12", "4c51945a6167ca06c0169e7a4ca5a8e7ffa3faba"]
+  "v16": ["16.3", "3bf9219f6ef5e943393e9430872e26184e92d1c6"],
+  "v15": ["15.7", "2092a6dcee794bb0cb17471bd964690dd7c7355f"],
+  "v14": ["14.12", "aa88bd536b48b22328aac748be0dcfff760135d0"]
 }