This is mostly in preparation for changes soon to be merged from PostgreSQL
8.4, commit a77eaa6a to be more precise. Currently GPDB's ExecInsert
uses ExecSlotFetch*() functions to get the tuple from the slot, while in
the upstream, it makes a modifiable copy with ExecMaterializeSlot(). That's
OK as the code stands, because there's always a "junk filter" that ensures
that the slot doesn't point directly to an on-disk tuple. But commit
a77eaa6a will change that, so we have to start being more careful.

This does fix an existing bug, namely that if you UPDATE an AO table with
OIDs, the OIDs currently change (github issue #3732). Add a test case for
that.

More detailed breakdown of the changes:

* In ExecInsert, create a writeable copy of the tuple when we're about
  to modify it, so that we don't accidentally modify an existing on-disk
  tuple. By calling ExecMaterializeSlot().

* In ExecInsert, track the OID of the tuple we're about to insert in a
  local variable, when we call the BEFORE ROW triggers, because we don't
  have a "tuple" yet.

* Add ExecMaterializeSlot() function, like in the upstream, because we now
  need it in ExecInsert. Refactor ExecFetchSlotHeapTuple to use
  ExecMaterializeSlot(), like in upstream.

* Cherry-pick bug fix commit 3d02cae3 from upstream. We would get that
  soon anyway as part of the merge, but we'll soon have test failures if
  we don't fix it immediately.

* Change the API of appendonly_insert(), so that it takes the new OID as
  argument, instead of extracting it from the passed-in MemTuple. With this
  change, appendonly_insert() is guaranteed to not modify the passed-in
  MemTuple, so we don't need the equivalent of ExecMaterializeSlot() for
  MemTuples.

* Also change the API of appendonly_insert() so that it returns the new OID
  of the inserted tuple, like heap_insert() does. Most callers ignore the
  return value, so this way they don't need to pass a dummy pointer
  argument.

* Add test case for the case that a BEFORE ROW trigger sets the OID of
  a tuple we're about to insert.

This is based on earlier patches against the 8.4 merge iteration3 branch by
Jacob and Max.