alter: initialize truncate system space lazily

Currently, space.create must insert an entry into 'truncate' system
space - if it does not, space.truncate won't work. This is incontinent,
as it makes it impossible to create spaces by simply inserting a tuple
into the 'space' system space via iproto API. So let's insert entries
into truncate space lazily, on the first space truncation.

Closes #2524