Change Vochain transaction nonce from random bytes to uint32

[jordipainan]

Like in this line: https://github.com/vocdoni/vaas-api/blob/master/vocclient/vocclient.go#L570 Setting an account infoURI requires the tx to provide a valid account nonce which is the autoincremental one not a random one like in the past. So the flow will be:

• GetAccount -> Provides the next nonce to use
• Create tx with the provided nonce
• Send tx

To take into account: Getting the nonce when a tx is pending to be mined will return the same nonce that is used in such tx. So as the mempool does not have ordering can be that a second tx sent with the same nonce is added into the storage before tx 1. This means that the first transaction will be invalidated.

[jordipainan]

cc: @emmdim

[jordipainan]

Done: 2758a373057c8733bea2ac56f9010703fac86fbb