* don't allow fkey based relations from/to a table that isn't tracked, fix#185
Check if remote table exist in metadata when creating foreign-key
based object relationship.
* add tests for adding object relation using fkey if remote table is untracked
* add 'on_conflict' condition to allow upsert mutation, closes#105
* check for empty unique or primary key constraints
* add 'on_conflict' condition test cases and introspection test case
* update 'conflict_action' enum values' description
