Federated Server functions

[Algunenano]

Add some new functions to manage a federated server.

This includes parts of the code from https://github.com/CartoDB/cartodb-postgresql/pull/373 and https://github.com/CartoDB/cartodb-postgresql/pull/381 and probably deprecates code recently added to CDB_ForeignTable.

As a WIP it still misses quite a bit of tests around corner cases (server / schema not existing, odd names...) and has some TODO's left (permissions, possible improvements) but it can be reviewed so we confirm this is the API we want to use.

Public functions:

• CDB_Federated_Server_List_Servers(server TEXT DEFAULT '%')
• CDB_Federated_Server_Register_PG(server TEXT, config JSONB)
• CDB_Federated_Server_Unregister(server TEXT)
• CDB_Federated_Server_List_Remote_Schemas(server TEXT)
• CDB_Federated_Server_List_Remote_Tables(server TEXT, remote_schema TEXT)
• CDB_Federated_Table_Register( server TEXT, remote_schema TEXT, remote_table TEXT, id_column TEXT, geom_column TEXT DEFAULT NULL, webmercator_column TEXT DEFAULT NULL, local_name NAME DEFAULT NULL )
• CDB_Federated_Table_Unregister( server TEXT, remote_schema TEXT, remote_table TEXT )
• CDB_Federated_Server_List_Registered_Tables( server TEXT, remote_schema TEXT )

[Algunenano]

Hope listings have some ORDER BY somewhere to avoid testability issues.

All listing functions use ORDER BY so it's covered.