Serialize ClusteringPrefixes in batches

Typically we will have very few clustering prefixes to serialize, however in theory they are not constrained (or are they, just to a very large number?). Currently we encode a fat header for all values up front (two bits per value), however those bits will typically be zero, and typically we will have only a handful (perhaps 1 or 2) of values.

This patch modifies the encoding to batch the prefixes in groups of up to 32, along with a header that is vint encoded. Typically this will result in a single byte per batch, but will consume up to 9 bytes if some of the values have their flags set. If we have more than 32 columns, we just read another header. This means we incur no garbage, and compress the data on disk in many cases where we have more than 4 clustering components.

I do wonder if we shouldn't impose a limit on clustering columns, though: If you have more than a handful merge performance is going to disintegrate. 32 is probably well in excess of what we should be seeing in the wild anyway.