Fix lookups in pg_{clear,restore}_{attribute,relation}_stats().

Presently, these functions look up the relation's OID, lock it, and
then check privileges.  Not only does this approach provide no
guarantee that the locked relation matches the arguments of the
lookup, but it also allows users to briefly lock relations for
which they do not have privileges, which might enable
denial-of-service attacks.  This commit adjusts these functions to
use RangeVarGetRelidExtended(), which is purpose-built to avoid
both of these issues.  The new RangeVarGetRelidCallback function is
somewhat complicated because it must handle both tables and
indexes, and for indexes, we must check privileges on the parent
table and lock it first.  Also, it needs to handle a couple of
extremely unlikely race conditions involving concurrent OID reuse.

A downside of this change is that the coding doesn't allow for
locking indexes in AccessShare mode anymore; everything is locked
in ShareUpdateExclusive mode.  Per discussion, the original choice
of lock levels was intended for a now defunct implementation that
used in-place updates, so we believe this change is okay.

Reviewed-by: Jeff Davis <pgsql@j-davis.com>
Discussion: https://postgr.es/m/Z8zwVmGzXyDdkAXj%40nathan
Backpatch-through: 18

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