xtrabackup: Add a buffered reader in front of xtrabackup's stdout pipe.
xtrabackup: Add a buffered reader in front of xtrabackup's stdout pipe.

The buffered reader implements WriteTo(), which allows io.CopyN() to
avoid allocating a new buffer every time.

Signed-off-by: Anthony Yeh <1cdc736ba6fd89dd9b69239aea62ddb9fd1482b1@planetscale.com>

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