Feature: Replace Mongogrant with Vault

[shyamd]

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Vault has done a lot upgrade it's capabilities since the vault store was first created. It would be nice to rebuild our VaultStore so that we can replace mongogrant with all its fickleness and brittleness with Vault.

Overall goals

• A single VaulStore class that encapsulates all the interaction with the Vault Server, including potentially storing the Vault access token
• access vault only on connect, create the corresponding Store, IE Atlas and MongoDB credentials become a MongoStore, and don't store authentication information
• Access to the underlying store should be completely opaque. IE you still run vault_store.query, not vault_store.connected_store.query etc.