Allow to reuse tuple_formats for ephemeral spaces

Since under heavy load with SQL queries ephemeral
spaces might be extensively used it is possible to run out
of tuple_formats for such spaces. This occurs because
tuple_format is not immediately deleted when ephemeral space is
dropped. Its removel is postponed instead and triggered only
when tuple memory is exhausted.
As far as there's no way to alter ephemeral space's format,
let's re-use them for multiple epehemral spaces in case
they're identical.

Closes #3924