[multi-test] Add staking and distribution module

[ueco-jb]

Closes #753

Implementation of Staking and Distribution modules in multitest package, which allows now to test staking and delegation.

[ethanfrey]

Nice start.
The two big TODO are obvious missing points you mentioned to me.
I designed this to be possible. Probably need 30 minutes in the morning to push such a commit that will compile and do that

[chipshort]

This now allows adding multiple validators and the rewards are calculated based on a configurable APR, the validator's commission and the elapsed time, so you can just change block.time and send the withdrawal message. I also implemented the unbonding period (tough only when undelegating, not when redelegating). To trigger the actual payout when unbonding, a sudo message has to be sent to process the unbonding queue.
Not sure if this is a good solution, but I couldn't come up with a better way.

Looks like tarpaulin flags a lot of lines that are covered.

[JakeHartnell]

Can't wait for this!

[uint]

The code looks fine! I can't easily verify this is precisely how staking/distribution works, so I'm taking that on faith.

[ethanfrey]

@ueco-jb can you add "Closes #753 " to the description, and maybe update it (I think it isn't WIP anymore)

[ueco-jb]

@chipshort In reward formula remember to subtract the validator commision
reward * (1 - commision)

[chipshort]

@chipshort In reward formula remember to subtract the validator commision reward * (1 - commision)

That should already be the case in calculate_rewards, albeit I calculated it in a slightly different way:

let commission = reward * validator_commission;

reward - commission