Add routines for marking buffers dirty efficiently

Add routines for marking buffers dirty efficiently

This commit introduces new internal bufmgr routines for marking shared
buffers as dirty:
* MarkDirtyUnpinnedBuffer()
* MarkDirtyRelUnpinnedBuffers()
* MarkDirtyAllUnpinnedBuffers()

These functions provide an efficient mechanism to respectively mark one
buffer, all the buffers of a relation, or the entire shared buffer pool
as dirty, something that can be useful to force patterns for the
checkpointer.  MarkDirtyUnpinnedBufferInternal(), an extra routine, is
used by these three, to mark as dirty an unpinned buffer.

They are intended as developer tools to manipulate buffer dirtiness in
bulk, and will be used in a follow-up commit.

Author: Nazir Bilal Yavuz <[email protected]>
Reviewed-by: Andres Freund <[email protected]>
Reviewed-by: Aidar Imamov <[email protected]>
Reviewed-by: Amit Kapila <[email protected]>
Reviewed-by: Joseph Koshakow <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Reviewed-by: Yuhang Qiu <[email protected]>
Reviewed-by: Xuneng Zhou <[email protected]>
Discussion: https://postgr.es/m/CAN55FZ0h_YoSqqutxV6DES1RW8ig6wcA8CR9rJk358YRMxZFmw@mail.gmail.com