Issue 51

Fixes #51:
* New field in `json_rpc` table: `one_row_flags :: boolean[]` - can be empty or length of `sql_queries` array. Each boolean in the array determines if single row constraint is checked for corresponding SQL query.
* If a query with `one_row_flags` returns non one row respond then transaction of the query is rollbacked and an RPC error is sent to the client. 
* Failed transactions on single row constraint are logged
* Invalid state of `one_row_flags :: boolean[]` is checked