More efficient log block checksums

Behavior of innodb_log_checksum_algorithm depending on its value is
mostly identical to innodb_checksum_algorithm, except that the former
applies to log rather than page checksums.

innodb_log_checksum_algorithm=none means that a constant value will be
written to log blocks instead of calculated checksum values and no
checksum validation will be performed on InnoDB/XtraBackup recovery, or
changed page tracking (if enabled).

innodb_log_checksum_algorithm=innodb (the default) means the default
InnoDB behavior -- a custom and inefficient algorithm is used to
calculate log checksums, but logs created with this option are
compatible with upstream MySQL and earlier Percona Server or XtraBackup
versions that do not support other log checksum algorithms.

innodb_log_checksum_algorithm=crc32 will use CRC32 for log block
checksums. Checksums will also benefit from hardware acceleration
provided by recent Intel CPUs.

Normally, XtraDB or XtraBackup will tolerate checksums created with
other algorithms than is currently specified with the
innodb_log_checksum_algorithm option. That is, if checksums don't match
when reading the redo log on recovery, the block is considered corrupted
only if no algorithm produces the value matching the checksum stored in
the log block header.

This can be disabled by prepending the value with the 'strict_' suffix,
e.g. 'strict_none', 'strict_crc32' or 'strict_innodb' will only accept
checksums created using the corresponding algorithms, but not the other
ones.

The XtraBackup support is covered by
https://blueprints.launchpad.net/percona-xtrabackup/+spec/support-innodb-log-checksum-algorithm