Change GracefulStop into a RO channel

[Freeaqingme]

Migrate never writes to the GracefulStop channel, so why not guarantee that by turning the chan into a ReadOnly channel?

In my case I already had such a channel available which happened to be RO, so for me it wasn't just a nice to have, but sort of a necessity :)

I believe this change does not break BC.

[Freeaqingme]

Apparently the CLI tooling does write directly to the chan. Coming to think of it, other people may do that as well, since it's an exported property. It may be a nice change for the next major version then (given that I also fix the test)?