txnbuild: provide simple account implementation

[ire-and-curses]

The interface txnbuild.Account defines the requirements for an account used by transactions and operations. Often, the workflow is to obtain an account using horizonclient, and then use this to build new transactions.

However, this is not always the case. It would be helpful to provide a minimal implementation SimpleAccount for situations where an account is not coming from a horizonclient call. This functionality is needed in friendbot, and the bridge server, for example. It would also simplify txnbuild test code, which currently has this funky helper method:

func makeTestAccount(kp *keypair.Full, seqnum string) hProtocol.Account {
	return hProtocol.Account{
		HistoryAccount: hProtocol.HistoryAccount{
			AccountID: kp.Address(),
		},
		Sequence: seqnum,
	}
}

@debnil has a minimal implementation here. We could move something like this to txnbuild. Also see the implementation in protocols/horizon:

// GetSequenceNumber returns the sequence number of the account,
// and returns it as a 64-bit integer.
func (a Account) GetSequenceNumber() (xdr.SequenceNumber, error) {
	seqNum, err := strconv.ParseUint(a.Sequence, 10, 64)

	if err != nil {
		return 0, errors.Wrap(err, "Failed to parse account sequence number")
	}

	return xdr.SequenceNumber(seqNum), nil
}

// IncrementSequenceNumber increments the internal record of the account's sequence
// number by 1. This is typically used after a transaction build so that the next
// transaction to be built will be valid.
func (a *Account) IncrementSequenceNumber() (xdr.SequenceNumber, error) {
	seqNum, err := a.GetSequenceNumber()
	if err != nil {
		return xdr.SequenceNumber(0), err
	}
	seqNum++
	a.Sequence = strconv.FormatInt(int64(seqNum), 10)
	return a.GetSequenceNumber()
}

• Implement minimal account struct matching Account interface
• Switch tests over to new account

[ire-and-curses]

Closed by #1271