Avoid some unnecessary block reads in WAL reader

Avoid some unnecessary block reads in WAL reader

When reading a new page internally and depending on the way the WAL
reader facility gets used by plugins, the current implementation of the
WAL reader may finish by reading a block multiple times while it is not
actually necessary as the requested data length may be equal to what has
been already read.  This can happen for any size, but is more likely to
happen at the end of a page.  This can cause performance penalties in
plugins which rely on the block reads to be purely sequential, zlib not
liking backward reads for example.  The new behavior also shaves some
cycles when doing recovery.

Author: Arthur Zakirov
Reviewed-by: Andrey Lepikhov, Michael Paquier, Grigory Smolkin
Discussion: https://postgr.es/m/2ddf4a32-517e-d6f4-d992-4a63b6035bfd@postgrespro.ru