Use read streams in pg_visibility

Hi,

I am working on using the read stream in pg_visibility. There are two
places to use it:

1- collect_visibility_data()

This one is straightforward. I created a read stream object if
'include_pd' is true because read I/O is done when 'include_pd' is
true. There is ~4% timing improvement with this change. I started the
server with the default settings and created a 6 GB table. Then run
100 times pg_visibility() by clearing the OS cache between each run.
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2- collect_corrupt_items()

This one is more complicated. The read stream callback function loops
until it finds a suitable block to read. So, if the callback returns
an InvalidBlockNumber; it means that the stream processed all possible
blocks and the stream should be finished. There is ~3% timing
improvement with this change. I started the server with the default
settings and created a 6 GB table. Then run 100 times
pg_check_visible() by clearing the OS cache between each run.

The downside of this approach is there are too many "vmbuffer is valid
but BufferGetBlockNumber(*vmbuffer) is not equal to mapBlock, so
vmbuffer needs to be read again" cases in the read stream version (700
vs 20 for the 6 GB table). This is caused by the callback function of
the read stream reading a new vmbuf while getting next block numbers.
So, vmbuf is wrong when we are checking visibility map bits that might
have changed while we were acquiring the page lock.
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Both patches are attached.

Any feedback would be appreciated.

--
Regards,
Nazir Bilal Yavuz
Microsoft