StorageEngine cannot index values over Number.MAX_SAFE_INTEGER

[jacque006]

Problem

When adding or retrieving an item using a StorageEngine, a value over Number.MAX_SAFE_INTEGER cannot be used as the itemID is a number. This is possible with Hubble as L1 ID values are uint256.

Solution

• Update itemID and other number values to be a BigNumberish in https://github.com/thehubbleproject/hubble-contracts/blob/master/ts/client/storageEngine/interfaces.ts#L9 and https://github.com/thehubbleproject/hubble-contracts/blob/master/ts/client/storageEngine/interfaces.ts#L16-L25
• Update memory and db implementations. Note for leveldb that we may have to specify a correct input/output encoding for BigNumber https://github.com/Level/encoding-down
• Remove extraneous existing toNumber conversions such as at https://github.com/thehubbleproject/hubble-contracts/blob/master/ts/client/database/stateEngine.ts#L23 to prevent over/underflow issues
• Add a test case which adds and retrieves an itemID larger than Number.MAX_SAFE_INTEGER to verify functionality

[jacque006]

Closing as @kautukkundan and I determined that current max ID (stateID, pubkeyID) of trees is (2^32) - 1, and JS has us covered up to (2^53) - 1. This issue can be re-opened/revisited if we substantially increase the ID space.