sql: Implement Tarantool support in `Clear` b-tree method (#2145).

     * Make `Clear` btree method use direct box_truncate().
     * Add testcase.

sql: Decode NULL promoting the pointer (#2145).

     * MsgPack decode of NULL in SQL connector didn't promote parse pointer.
     Add proper promotion. Add test.

sql: Remove normalize_iter_type() from sql.c

Remove the function definition. Use a similar function from index.h

sql: Remove get_space_id() from sql.c

Use SQLITE_PAGENO_TO_SPACEID/SQLITE_PAGENO_TO_INDEXID instead.