AIP #61 AdEx v5: Adapter - total spendable amount

[elpiel]

https://github.com/AdExNetwork/aips/issues/61

Rust reference:

Issue: AmbireTech/adex-validator-stack-rust#380
PRs: AmbireTech/adex-validator-stack-rust#385

https://github.com/AdExNetwork/aips/issues/61

function to calculate total spendable amount; this will be used by sentry to update it on request

Function: adapter.get_deposit(channel, spender) -> Deposit {total, stillOnCreate2}
This will be stored in the DB as {channelId, spender, total, stillOnCreate2} later on, unique by channelId, spender

• outpace.deposited[channelId][spender] ( outpace.deposited(channelId, spender) )
• stillOnCreate2 - erc20.balanceOf(counterfactualDepositAddr) this should be passed by the channel.token
  create2 address: https://github.com/AdExNetwork/adex-protocol-eth/blob/identity-tests/contracts/IdentityFactory.sol#L39-L41
• Configurable minimum deposit (per whitelisted stablecoin)
• store both values so that we know stillOnCreate2
• update adex-protocol-eth to get access to https://github.com/AdExNetwork/adex-protocol-eth/blob/master/abi/ERC20.json