Concurrency issues when reading and writing retain messages

[yh742]

Hi there,

I'm trying to embed the broker into an application, but I've noticed while testing the library, the race detector goes off when publishing and subscribing to retain messages at the same time. I believe the issue is that there's no locks here:

https://github.com/mochi-co/mqtt/blob/7bd7bd5087c40f96015a61d01ce7ae4e6951c807/topics.go#L426-L428

And there's a lock here:

https://github.com/mochi-co/mqtt/blob/7bd7bd5087c40f96015a61d01ce7ae4e6951c807/topics.go#L354-L377