POC: Lock updated tuples in tuple_update() and tuple_delete()

Hackers,

When working in the read committed transaction isolation mode
(default), we have the following sequence of actions when
tuple_update() or tuple_delete() find concurrently updated tuple.

1. tuple_update()/tuple_delete() returns TM_Updated
2. tuple_lock()
3. Re-evaluate plan qual (recheck if we still need to update/delete
and calculate the new tuple for update)
4. tuple_update()/tuple_delete() (this time should be successful,
since we've previously locked the tuple).

I wonder if we should merge steps 1 and 2. We could save some efforts
already done during tuple_update()/tuple_delete() for locking the
tuple. In heap table access method, we've to start tuple_lock() with
the first tuple in the chain, but tuple_update()/tuple_delete()
already visited it. For undo-based table access methods,
tuple_update()/tuple_delete() should start from the last version, why
don't place the tuple lock immediately once a concurrent update is
detected. I think this patch should have some performance benefits on
high concurrency.

Also, the patch simplifies code in nodeModifyTable.c getting rid of
the nested case. I also get rid of extra
table_tuple_fetch_row_version() in ExecUpdate. Why re-fetch the old
tuple, when it should be exactly the same tuple we've just locked.

I'm going to check the performance impact. Thoughts and feedback are welcome.

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Regards,
Alexander Korotkov