Currently, even though it takes in a `Logger` interface,
`log.SetDefualt` enforces that the concrete type of the provided logger
is `*logger` because:
1. in `init` `root.Store` is called with a `*logger`
2. `atomic.Value` panics if the concrete type provided in `Store` is not
consistent across calls.
([ref](https://pkg.go.dev/sync/atomic#Value.Store))
> All calls to Store for a given Value must use values of the same
concrete type.
This PR changes to use `sync.RWMutex` and adds a test that panics on
`master`.
