callback<Function> 当剩余的数据被 flush 后的回调函数。
当流结束时，这些额外的数据需要被 flush 才算完成压缩。
当 flush 操作完成时，必须调用
callback<Function> A callback function (optionally with an error argument and data) to be called when remaining data has been flushed.
This function MUST NOT be called by application code directly. It should be
implemented by child classes, and called by the internal
In some cases, a transform operation may need to emit an additional bit of
data at the end of the stream. For example, a
zlib compression stream will
store an amount of internal state used to optimally compress the output. When
the stream ends, however, that additional data needs to be flushed so that the
compressed data will be complete.
Transform implementations may implement the
method. This will be called when there is no more written data to be consumed,
but before the
'end' event is emitted signaling the end of the
transform._flush() implementation, the
may be called zero or more times, as appropriate. The
callback function must
be called when the flush operation is complete.
transform._flush() method is prefixed with an underscore because it is
internal to the class that defines it, and should never be called directly by