tube:create() should check that a space with this name exists

[kostja]

If a space with the same name as tube name exists, queue module does not check that this space schema has structure which is different from the desired one, and goes on (if_not_exists clause is set to true).
This is error prone, since later on such queue will malfunction.

Check that the space used for queue actually is created with queue:create and complain if not.

[Totktonada]

Should be resolved with PR #109.