Implement Claim Takeover

[0xferit]

Let users take over a claim by refunding the bounty deposit of the old owner and escrowing the bounty deposit of the new owner.

1. The new deposit should not be less than the old one.
2. When rewards are implemented, the new owner should be accepting a lesser reward.

In a sense, the new owner puts a bid for a claim for less reward: "I'm willing to collateralize this claim for less reward".