Adding vesting logic to claims

[shawntabrizi]

This adds vesting logic to the claims module. Fixes #612.

• There is a new storage item that can be configured at genesis.

/// Vesting schedule for a claim.
/// First balance is the total amount that should be held for vesting.
/// Second balance is how much should be unlocked per block.
/// The block number is when the vesting should start.
Vesting get(vesting) config(): map EthereumAddress => Option<(BalanceOf<T>, BalanceOf<T>, T::BlockNumber)>;

• When a user submits a claim, we check:

  • There actually exists a claim
  • If the claim has a vesting schedule attached, that we can apply it to the account that the user wants. This means accounts that already have a vesting schedule applied to them cannot be the target of a claim with a vesting schedule attached. The runtime will return an error and nothing is changed.

• If all checks pass, we clean up the Claims and Vesting storage items, then try to transfer the funds.

• mint_claim is updated to support an optional vesting schedule that will be written to storage if provided.

• Tests updated.

[shawntabrizi]

Please do not merge until polkadot-master branch is updated again :)

It needs paritytech/substrate#4257

[gavofyork]

will need a runtime bump.