Store sparse doc values more efficiently

For both NUMERIC fields and ordinals of SORTED fields, we store data in a dense way. As a consequence, if you have only 1000 documents out of 1B that have a value, and 8 bits are required to store those 1000 numbers, we will not require 1KB of storage, but 1GB.

I suspect this mostly happens in abuse cases, but still it's a pity that we explode storage requirements. We could try to detect sparsity and compress accordingly.