Allow using copy_file_range in write_reconstructed_file

Allow using copy_file_range in write_reconstructed_file

This commit allows using copy_file_range() for efficient combining of
data from multiple files, instead of simply reading/writing the blocks.
Depending on the filesystem and other factors (size of the increment,
distribution of modified blocks etc.) this may be faster than the
block-by-block copy, but more importantly it enables various features
provided by CoW filesystems.

If a checksum needs to be calculated for the file, the same strategy as
when copying whole files is used - copy_file_range is used to copy the
blocks, but the file is also read for the checksum calculation.

While the checksum calculation is rarely needed when cloning whole
files, when reconstructing the files from multiple backups it needs to
happen almost always (the only exception is when the user specified
--no-manifest).

Author: Tomas Vondra
Reviewed-by: Thomas Munro, Jakub Wartak, Robert Haas
Discussion: https://postgr.es/m/3024283a-7491-4240-80d0-421575f6bb23%40enterprisedb.com