Support CEP-16 sharded repodata

[dholth]

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What is the idea?

CEP-16 defines a new repodata format, storing an index and per-project files. This helps with network and disk bandwidth. conda-index should generate this format.

Work has been started in this pull request.

In general, features from https://github.com/dholth/conda-index/blob/sharded-repodata/conda_index/index/shards.py should be merged into the CLI as a --write-shards option, into https://github.com/dholth/conda-index/blob/sharded-repodata/conda_index/index/__init__.py and into https://github.com/dholth/conda-index/blob/sharded-repodata/conda_index/index/sqlitecache.py

We should track the feature's performance over time. So far it takes about the same amount of time to generate shards, versus monolithic repodata.json. We will have to continue to generate both at the same time.