Revert semantic of session accessors

In an attempt to make memory segments more securable, we have tweaked session accessors to return non-closeable views.

While this effectively prevents clients from closing a session if they only have a memory segment, which might be desirable in some cases, in the general case, and especially in the case of one-off memory segments, associated with a single scope, going down this path is problematic, and introduce asymmetries in the API.

At the same time, close() is just one of the things that an API might want to protect against; clients might allocate memory in the API scope, or make it non-closeable by adding a close action that lasts forever.

In other words, the measures we took harm some use cases without achieving the desired effects of making memory segments more shareable across clients.