The check_param and check_param_table Lua helpers are defined in
box/lua/schema.lua but used across the whole code base. The problem is
we can't use them in files that are loaded before box/lua/schema.lua,
like box/lua/session.lua. Let's move them to a separate source file
lua/utils.lua to overcome this limitation. Also, let's add some tests.

NO_DOC=refactoring
NO_CHANGELOG=refactoring

(cherry picked from commit d8d267c5)