MSgCreateValidator and MsgUpdateValidator race condition

[RiccardoM]

Bug description

Currently there is a possible bug while handling MsgCreateValidator and MsgUpdateValidator transactions that is due to a race condition. Suppose that a validator performs the following actions:

1. At height X, it sends a MsgCreateValidator transaction with commission of 10%
2. At height X+10, it sends a MsgUpdateValidator transaction updating other details but not the commission.

Suppose also that the MsgUpdateValidator is parsed before the MsgCreateValidator transaction. What would happen is the following:

1. BDJuno parses the MsgUpdateValidator and, since the commission is not set, it stores a commission of 0%
2. BDJuno parses the MsgCreateValidator with the correct commission, but since the latest commission as a higher height, it is never updated and the correct commission (which should be 10%) is never stored.

Expected behavior

The MsgCreateValidator should store the correct infos, and the MsgUpdateValidator should not update the wrong fields.

Possible solution

We might investigate whether is worth using some custom values to identify these cases. For example setting a commission of -1 for those validators who have not their commission set yet. Then the MsgCreateValidator will update those values accordingly.