Fix insertSelect() with IGNORE in PostgreSQL

Fix insertSelect() with IGNORE in PostgreSQL

I0e6a9e6d overlooked the special handling PG needs (prior to 9.5 anyway)
to properly emulate MySQL's IGNORE option when delegating to the parent
implementation.

For now, then, don't use the native implementation in PG when IGNORE is
specified. Instead, fall back to the non-native implementation that does
a select() then an insert() where PG can handle the IGNORE properly.

In the future we might use the ON CONFLICT DO NOTHING clause added in PG
9.5 to be able to do native insertSelect() with IGNORE (and to better
handle multi-row insert() with IGNORE, and we could use the related ON
CONFLICT DO UPDATE to implement upsert()). All that is left for a future
patch.

Change-Id: I7987d59580543de03d5c6a5ed7fa3ce551ac12f3