Staking renaming

[rigelrozanski]

in the interest of changing APIs prelaunch (sooner than later) as I've been reviewing staking there are a few renames which I think we ought to update:

• MsgBeginUnbonding -> MsgUndelegate
  • Begin unbonding doesn't really make sense because the validator may be unbonding (in which case the tokens have already begun unbonding) or unbonded (tokens do not enter into an UnbondingDelegation object)
• field name Delegation -> Amount (these are type sdk.Coins) in MsgCreateValidator and MsgDelegate
  • Delegation is an ambiguous field name because of the Delegation type in x/stake/types/delegation.go
• Validator.UnbondingMinTime -> Validator.UnbondingCompletionTime stay consistent with unbonding delegation object/thinking

CC @cwgoes @jackzampolin @faboweb

[jackzampolin]

I like both of these changes. Will this also necessitate any changes on the query side? Would be interested to hear @faboweb's thoughts from the Voyager side of things.

[faboweb]

I agree with those changes. 👍

[cwgoes]

All listed changes look fine to me.