Reduce size of buffer, stringBuffer and tape.

[ZhaiMo15]

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In class JsonValue, the default size of buffer and stringBuffer, as well as long[] tape in class Tape, is 34M. But in practice it's not necessary. This patch reduce the size of them from 34M to its actual size.

[ZhaiMo15]

I believe in future, the JsonValue might be deep copied, thus the size of byte[] and long[] is important.