Add ignore on duplicate key to insert query

Problem

Currently there is no way to use an insert query with ignore when duplicate key is found.

Proposed resolution

I think the best way to implement it is the following:

• Update the base insert class with a new method to set a boolean that will enable this functionality.
• Update the MySQL insert class to use INSERT IGNORE when the boolean is set.
• Update the PostgreSQL insert class to use INSERT ... ON CONFLICT DO NOTHING when the boolean is set. This brings the requirement of having version 9.5 or newer. Is this a problem? Could add an extra requirement to this specific functionality.
• Update the SQLite insert class to use INSERT OR IGNORE when the boolean is set.

Remaining tasks

1. Discuss the proposed solution
2. Write a patch
3. Review

I have also some idea's about duplicate key update solutions, but for now lets first handle this ticket.