Up to this point, the is_temporary flag has been set for surrogate space
definitions used to transfer meta-information during compilation stage
of DML queries (CREATE TABLE/INDEX etc).

At some point, it became possible to use spaces created in Lua in
SQL statements. Since it is allowed to create temporary spaces in
Lua, not all temporary spaces in SQL are surrogate wrappers. To separate
real temporary spaces (i.e. which came from space cache) from surrogate
ones, is_ephemeral flag is now set in such spaces instead of
is_temporary. Note that there can't be mess between these flags since
ephemeral spaces can exist only during execution of VDBE bytecode.
Also note that flag is required only for debugging facilities.

Close #4139