add ChangeSlaveId api

[wumingyu12]

No description provided.

[andig]

+1 for this PR. I would prefer a different name though, maybe something like SetSlaveId or SetTargetSlave. With API available you could hide SlaveId as well which would be a breaking change.
It might also make sense to panic if SlaveId is not initialised with a valid value?

[andig]

I'd still greatly welcome this getting merged as it would get rid of ugly duck typing like this:

if handler, ok := q.handler.(*modbus.RTUClientHandler); ok {
	handler.SlaveId = deviceid
} else if handler, ok := q.handler.(*modbus.TCPClientHandler); ok {
	handler.SlaveId = deviceid
}

[andig]

@wumingyu12 would you mind adding Timeout to the API in the same way (getter, setter)? I think its also used throughout the handlers and deserves the same api exposure?