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PXB-1626: Support encrypted redo and undo logs

This patch adds support for encrypted redo and undo logs.

Since redo and undo logs are just a special case of InnoDB
tablespaces, changes needed to support them are minimal:

• xtrabackup now recognises `--innodb_redo_log_encrypt' and
  `innodb_undo_log_encrypt' server variables. Values of these
  variables are saved to `backup-my.cnf' and used for prepare.

• instead of using a keyring plugin for the case when
  `--transition-key' is specified, xtrabackup will look for
  encryption key in the `xtrabackup_keys' file.

• since encrypted redo log is decrypted by `recv_read_log_seg',
  xtrabackup will encrypt it before writing to `xtrabackup_logfile'.