Ignore nextOid when replaying an ONLINE checkpoint.

The nextOid value is from the start of the checkpoint and may well be stale
compared to values from more recent XLOG_NEXTOID records.  Previously, we
adopted it anyway, allowing the OID counter to go backwards during a crash.
While this should be harmless, it contributed to the severity of the bug
fixed in commit 0408e1ed, by allowing duplicate TOAST OIDs to be assigned
immediately following a crash.  Without this error, that issue would only
have arisen when TOAST objects just younger than a multiple of 2^32 OIDs
were deleted and then not vacuumed in time to avoid a conflict.

Pavan Deolasee

Discussion: https://postgr.es/m/CABOikdOgWT2hHkYG3Wwo2cyZJq2zfs1FH0FgX-=h4OLosXHf9w@mail.gmail.com