Sharded store

[ashwanthkumar]

For some use-cases where we can't have one large database (like RocksDB) for storing multi-terrabyte data. It would be great to have sharding of the Store and a wrapper store that manages it for us automatically.

ShardedStore extends Store, so it's a drop-in replacement for all of the existing stores and you would start having multiple shards automatically. If the existing store already has some data, they might have to be re-played or converted to the newer format. I would have written a tool for migration but since we still don't expose the scan interface on store, it's not possible to write a generic utility that can do this for us.

@brewkode Please check this out.

[codecov-io]

Current coverage is 64.44% (diff: 63.63%)

Merging #53 into master will increase coverage by 0.57%

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+ Hits            297        310    +13   
+ Misses          126        123     -3   
- Partials         42         48     +6   

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[ashwanthkumar]

Merging this since we need it for our finder benchmarks. Please share any feedback, I'll fix them in a separate PR.