sstables: change default buffer size to 128k
sstables: change default buffer size to 128k

Example run of perf_sstable_index:

 64k: 1401296.23 +- 5461.20 partitions / sec (30 runs, 1 concurrent ops)
128k: 1459283.89 +- 6674.87 partitions / sec (30 runs, 1 concurrent ops)

This is 4 % higher on an 0.45 % error

For larger buffers, like 256k, this doesn't yield a consistent gain, sometimes
yielding a loss.

Signed-off-by: Glauber Costa <f8b79557d19b40666ebe336545a0b6178af65685@cloudius-systems.com>

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