x/stake: Add global redelegation/unbonding index

[ValarDragon]

Currently this function: https://github.com/cosmos/cosmos-sdk/blob/develop/x/stake/keeper/key.go#L158 will have collisions at the same key. e.g.
Delegator has 10 steak bonded to validator A.
Delegator rebonds 3 steak to validator B
Delegator rebonds 5 steak to validator B - Issue here, the key is already in use.

One simple fix is to add an additional component to that key derivation, by appending bytes for a global delegation index, and then incrementing the global delegation index. Anything trying to access that redelegation can iterate over all the different nonces.

From a discussion with @cwgoes

[rigelrozanski]

This also applies to unbonding

Note I don't think that the user should be required to track these nonces, the command should just pick up from the most relevant object (if possible) - going to think about this one a bit more

[rigelrozanski]

I also wanted to note that in adding this we need some slightly more complex backend logic for processing unbonding-delegations or redelegations where there are multiple objects on the same routes. Related: #1624