Variable min deposit size to promote consistent # of vals

Issue

Defining an explicit, unchanging value for the minimum size of deposits is risky because we do not have a clear idea as to how many validators will show up. Instead, we want to define a base deposit value and have the minimum a validator needs to deposit scale as the number of validators scales.

Proposed Implementation

• Track number of active validators with a new contract variable num_validators set to 0 at contract creation
• When validator sends deposit assert msg.value >= max(self.min_deposit_size, num_validators)
• Increment num_validators when a new validator sends a deposit
• Decrement num_validators when a validator submits a logout or is slashed (if they haven’t logged out already)

[djrtwo]

I exposed the method current_min_deposit_size as a public method in the PR. This should probably suffice, right?

https://github.com/ethereum/casper/pull/98/files#diff-5907966b02ce3d72c8259ba4206c1338R302

[ChihChengLiang]

PR is welcome, remember to fix this comment in the PR too. That comment only makes sense in the dynamic min deposit size context.