Support Snappy compression

[karalabe]

I'm unsure about this one, mostly a question. We could in theory add under-the-hood snappy compression, but maybe that would break the bucket-sorting a bit, at least for blob txs. If the originally stable sizes become slightly randomized, it might end up with weird bucket sizes. So might be a moot point, just a thought maybe for other use cases (i.e. optional).

[holiman]

Would that be a per-database option? I suppose it would be. So no need to signal compression in the key