DecimalSerializer.toString() can be used as OOM attack

Currently, in DecimalSerializer.toString(value), it uses BigDecimal.toPlainString() which generates huge string for large scale values.

BigDecimal d = new BigDecimal("1e-" + (Integer.MAX_VALUE - 6));
d.toPlainString(); // oom

Propose to use BigDecimal.toString() when scale is larger than 100 which is configurable via -Dcassandra.decimal.maxscaleforstring

The code should apply cleanly to 3.0+.