sql: ON CONFLICT works correctly

- ON CONFLICT work checked:
Works correctly (not including ROLLBACK, because sql transactions
are not implemented now)
- Tests for ON CONFLICT implemented

test/sql/sql-onconflict.result

+test_run = require('test_run').new()
+---
+...
+-- Create space
+box.sql.execute("CREATE TABLE t (id INTEGER PRIMARY KEY, v INTEGER UNIQUE ON CONFLICT ABORT)")
+---
+...
+box.sql.execute("CREATE TABLE q (id INTEGER PRIMARY KEY, v INTEGER UNIQUE ON CONFLICT FAIL)")
+---
+...
+box.sql.execute("CREATE TABLE p (id INTEGER PRIMARY KEY, v INTEGER UNIQUE ON CONFLICT IGNORE)")
+---
+...
+box.sql.execute("CREATE TABLE e (id INTEGER PRIMARY KEY, v INTEGER NOT NULL ON CONFLICT REPLACE DEFAULT 1337)")
+---
+...
+-- Insert values and select them
+box.sql.execute("INSERT INTO t values (1, 1), (2, 2), (3, 1)")
+---
+- error: 'UNIQUE constraint failed: t.v'
+...
+box.sql.execute("SELECT * FROM t")
+---
+- - [1, 1]
+  - [2, 2]
+...
+box.sql.execute("INSERT INTO q values (1, 1), (2, 2), (3, 1)")
+---
+- error: 'UNIQUE constraint failed: q.v'
+...
+box.sql.execute("SELECT * FROM q")
+---
+- - [1, 1]
+  - [2, 2]
+...
+box.sql.execute("INSERT INTO p values (1, 1), (2, 2), (3, 1), (4, 5)")
+---
+...
+box.sql.execute("SELECT * FROM p")
+---
+- - [1, 1]
+  - [2, 2]
+  - [4, 5]
+...
+box.sql.execute("INSERT INTO e values (1, 1), (2, 2), (3, 1)")
+---
+...
+box.sql.execute("SELECT * FROM e")
+---
+- - [1, 1]
+  - [2, 2]
+  - [3, 1]
+...

test/sql/sql-onconflict.test.lua

+test_run = require('test_run').new()
+
+-- Create space
+box.sql.execute("CREATE TABLE t (id INTEGER PRIMARY KEY, v INTEGER UNIQUE ON CONFLICT ABORT)")
+box.sql.execute("CREATE TABLE q (id INTEGER PRIMARY KEY, v INTEGER UNIQUE ON CONFLICT FAIL)")
+box.sql.execute("CREATE TABLE p (id INTEGER PRIMARY KEY, v INTEGER UNIQUE ON CONFLICT IGNORE)")
+box.sql.execute("CREATE TABLE e (id INTEGER PRIMARY KEY, v INTEGER NOT NULL ON CONFLICT REPLACE DEFAULT 1337)")
+
+-- Insert values and select them
+box.sql.execute("INSERT INTO t values (1, 1), (2, 2), (3, 1)")
+box.sql.execute("SELECT * FROM t")
+
+box.sql.execute("INSERT INTO q values (1, 1), (2, 2), (3, 1)")
+box.sql.execute("SELECT * FROM q")
+
+box.sql.execute("INSERT INTO p values (1, 1), (2, 2), (3, 1), (4, 5)")
+box.sql.execute("SELECT * FROM p")
+
+box.sql.execute("INSERT INTO e values (1, 1), (2, 2), (3, 1)")
+box.sql.execute("SELECT * FROM e")