Introduce ephemeral spaces

Ephemeral spaces are invisible via public API and they are not
persistent. They are needed solely to do some transient calculations.

To create an ephemeral space, use space_new_ephemeral() function,
which takes a space definition and a list of index definitions as
arguments. A space created with this function may be accessed and
modified via internal API (space_execute_replace(), index_get(),
etc).

Currently, only the memtx engine supports ephemeral spaces.

Closes #2776