Compress TDigest instances more aggressively.

TDigest instances are expected to track about 5*compression centroids. Yet they
get compressed only when the number of centroids grows larger than
100*compression.

I did some experiments with lower thresholds, and it seems to behave rather
well. In particular, with a threshold of 20*compression (computed empirically),
I get:
 1. a slowdown of ~30% for sequential points (but memory usage is reduced by 5x)
 2. a speedup that can be significant (especially for ArrayDigest, I have seen
    speedups up to 50%) for data distributions whose amplitude is slowly
    expanding.

All in all, I think lowering the threshold to 20*compression would be a better
trade-off?
Merge pull request #23 from jpountz/more_aggressive_compression

Compress TDigest instances more aggressively.