sql: database should be locked for modifications during some query execution

This case was found in table-14.2 test.
You can catch main idea from this code:

local json = require("json")
box.cfg{}
box.sql.execute("create table t1(a primary key);")
box.sql.execute("create table t2(a primary key);")
box.sql.execute("insert into t2 values(1);")
box.sql.execute("insert into t2 values(2);")

local function drop_t1()
  local r, msg = pcall( function () box.sql.execute("DROP TABLE t1;") end)
  return msg
end
box.internal.sql_create_function("drop_t1", drop_t1)

local result = box.sql.execute("SELECT *, drop_t1() from t2;")
print(json.encode(result)) -- [[1,null],[2,"no such table: t1"]]

the result is [[1,null],[2,"no such table: t1"]] instead of
[[1,"database table is locked"],[2,"database table is locked"]]

It is even possible to drop t2 while selecting data from it.