sql: introduce pragma sql_default_engine

Part of #2199.

@TarantoolBot document
Title: new pragma sql_default_engine
Now it is allowed to create vinyl spaces using special pragma
setting default engine for SQL requests.
This config is stored in user_session() and affect *only*
tables been created with SQL.
Example:
\set language sql
pragma sql_default_engine='vinyl';
CREATE TABLE t3(a primary key,b,c);