Review and update visibility levels in Storage

[ikopylov]

Too many unnecessary pub(crate) usage (https://github.com/qoollo/pearl/blob/master/src/storage/core.rs).
Looks like ioring should be totally private (https://github.com/qoollo/pearl/blob/master/src/storage/core.rs#L55)
config and safe can have readonly access, so they should be exposed through the getter and declared as private (https://github.com/qoollo/pearl/blob/master/src/storage/core.rs#L52).
Same for fields of Safe: they should not be available for updated outside (https://github.com/qoollo/pearl/blob/master/src/storage/core.rs#L63). From the architecture point of view, Safe should not visible at all, because it is an integral part of Inner<K> protected by lock (this is discussable).