Use heap buffers for Unpooled.copiedBuffer()

Related issue: #2028

Motivation:

Some copiedBuffer() methods in Unpooled allocated a direct buffer.  An
allocation of a direct buffer is an expensive operation, and thus should
be avoided for unpooled buffers.

Modifications:

- Use heap buffers in all copiedBuffer() methods

Result:

Unpooled.copiedBuffers() are less expensive now.