Make Tarantool spaces synchronous again

[ligurio]

As described in documentation synchronous replication can be enabled per-space using the is_sync option. This option was missed after switching from Lua to SQL in tests.

(was lost in 3214905)

It is not possible to change is_sync using SET SESSION in SQL:

unix/:/var/run/tarantool/jepsen.control>  box.space._session_settings:select()
---
- - ['error_marshaling_enabled', false]
  - ['sql_default_engine', 'memtx']
  - ['sql_defer_foreign_keys', false]
  - ['sql_full_column_names', false]
  - ['sql_full_metadata', false]
  - ['sql_parser_debug', false]
  - ['sql_recursive_triggers', true]
  - ['sql_reverse_unordered_selects', false]
  - ['sql_select_debug', false]
  - ['sql_vdbe_debug', false]
...

But it is possible to create Lua function in SQL and use box.space.j:alter{is_sync = true} in it.