improve defaultCodec.Decode

[Abioy]

reduce memory copy times and reuse map objects in ttheader decode.

[YangruiEmma]

Thanks @Abioy. But the buffer is reused, it must be copied here or it will lead panic.

[Abioy]

Thanks @Abioy. But the buffer is reused, it must be copied here or it will lead panic.

@YangruiEmma Yes, buffer was copied in line 266 of header_codec.go within #122 .

[YangruiEmma]

Thanks @Abioy. But the buffer is reused, it must be copied here or it will lead panic.

@YangruiEmma Yes, buffer was copied in line 266 of header_codec.go within #122 .

From the perf test, read modification doesn't promote performance obviously. But the map reuse may be a point, this modification can be kept if the performance result is better.