Add tuple_ref() for a key_def tuple in box_truncate()
Add tuple_ref() for a key_def tuple in box_truncate()

A temporary tuple from key_def_tuple_update_lsn() will be
garbage-collected on parallel wal_write().

Fixes #2078

rtsisyk/tarantool,rtsisyk/tarantool,rtsisyk/tarantool,rtsisyk/tarantool