Use `AccAddress` instead of `ValAddress` for operator address

[keruch]

Use AccAddress instead of ValAddress for the operator address in x/sequencer messages.

[keruch]

We used the sequencer operator address as ValAddress in RDK historically, but in the Hub the sequencer operator address is AccAddress. Now, after relayer and whitelisted addr refactoring, we use the Hub as a source of truth, so address formats are jumbled.

I cleaned it up in the following way:

• In the Hub we use AccAddress everywhere.
• In dymint consensus messages, we just forward the settlement addr as a string without any conversion. so it's AccAddress bech32 encoded string from the Hub, eg dym1abcdefghigklmnop
• In RDK we keep using ValAddress as the operator address everywhere since in RDK we operate staking validators and their operator addr is ValAddress. When we get a consensus message, we extract raw bytes from the Hub's bech32 and then convert them to an RDK-prefixed bech32 val address to use it in the store.

So operator address:

• In the Hub: AccAddress
• In RDK: ValAddress

It also applies to MsgUpdateWhitelistedRelayers and MsgUpdateRewardAddress. In the Hub the signer is AccAddress, in RDK ValAddress, though in theory in RDK these messages will be unused.