sql: check read access while executing SQL query

Since SQL front-end is not using box API,
no checkes for read access are performed by VDBE engine.
Add check to IteratorOpen op-code to make sure that read
privilege exists for given space.
Note, that there's is no need to perform DML/DDL checkes as
they're performed by Tarantool's core.

@TarantoolBot document
Title: Document behaviour of SQL in presence of
read access restrictions. Need to clarify, that
if there's no read access to the space, then not
only SELECT statements will fail, but also those DML
which implies reading from spaces indirectly, e.g.:
  UPDATE t1 SET a=2 WHERE b=3;

Closes #2362