Memory storage should be persistent during process lifetime

[burmecia]

The memory storage currently is not identified by its URI, that is, each time repo.open is called a new memory storage is created regardless the URI. An in-memory 'persistent' storage should be used as back-end of memory storage, so that an URI specific memory storage can be opened multiple times during process lifetime.

For example, the 2 open calls below should open the same memory storage.

{
  let repo = RepoOpener::new().create(true).open("mem://foo", "pwd");
}
{
  let repo = RepoOpener::new().create(true).open("mem://foo", "pwd");
}