txm: skip ephemeral spaces while positioning iterator

In tree_iterator_start() it was assumed that iterator always contains
valid space id. However, ephemeral spaces are known to have zero space
id. So in case we are starting iterator which belongs to ephemeral
space, we can't simply find that space in space cache. Moreover, we
don't need to track ephemeral spaces in MVCC at all since they can be
accessed only pointers and their lifespan is restricted by SQL query
execution. So let's skip any MVCC-related routine while starting an
iterator.

Closes #6095