Provide ReadRecentBuffer() to re-pin buffers by ID.

Provide ReadRecentBuffer() to re-pin buffers by ID.

If you know the ID of a buffer that recently held a block that you would
like to pin, this function can be used check if it's still there.  It
can be used to avoid a second lookup in the buffer mapping table after
PrefetchBuffer() reports a cache hit.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ4VJN8ttxScUFM8dOKX0BrBiboo5uz1cq=AovOddfHpA@mail.gmail.com