Improve performance of file_checksum()

Improve performance of file_checksum()

Previously files were hashed in blocks of CSUM_CHUNK (64) bytes. This
causes significant overhead. The CSUM_CHUNK define cannot be changed as
md5.c depends on it, but there is no obvious reason to use it in
file_checksum(). By using CHUNK_SIZE (32 kB) instead, in some test
cases throughput more than doubles.