#36 Global constraints

[Michael Paquier]

Postgres-XC has now some limitations with constraints.
A constraint on a column or an inheritance can only be created if the constraint can be safely checked on remote node, for example:
- child and parent relations need to have similar distribution column types for REFERENCES or the parent needs to be a replicated table.

XC needs an additional mechanism to be able to fetch on Coordinator a tuple based on ctid and xc_node_id to be able to check constraint on Coordinator. Those kind of mechanisms are also used by triggers.