Create a Shard Interface/Struct With Useful Receiver Methods in Client Package

[rauljordan]

Hey all,

To proceed with the shard local state storage, I am creating this issue and a PR shortly to include a Shard interface/struct in our client package. This interface will have a lot of useful receiver methods for checking availability, interfacing with the shardDB backend, and fetching collations/headers from within a certain shard.

Context

This was worked on in Py-EVM ethereum/py-evm#570 and would also serve as a useful wrapper class to thin out the code we would otherwise have to write for notaries/proposers.

IMO we need this in order to proceed, as the next steps would be to define the shardDB as perhaps a sparse merkle tree wrapped around levelDB or badgerDB.

This is being worked on in #100.

[rauljordan]

Closed by #100