Events API: Two meetups returned for same date in same city

[dd32]

The Adelaide meetup group is seeing some odd results in the events API:

​http://api.wordpress.org/events/1.0/?location=Adelaide,%20SA
​https://www.dropbox.com/s/sdvqegjdm7lb6tq/Screenshot%202017-08-07%2011.42.30.png?dl=0

They use a monthly-repeating meetup, entitled WordPress Adelaide Monthly Meetup, and then edit the title to be WordPress Adelaide August Meetup.

Looking in the database, theres two source_id's: zdxnhlywlblb & hfllbnywlblb - it looks like when they edit the meetup out of the series it creates a new meetup, and the WordPress.org ingester fails to realise it already had that in the DB (because the source_id changed, but the URL stayed the same)

Looking at the database, it looks like this has happened in a few other locations as well. Perhaps we should consider keying it unique by type, url in addition to type, source_id.

[ocean90]

Related: #2720

[iandunn]

Also reported in ​#wp41935.

[iandunn]

Related #3321

[iandunn]

The changing IDs might be ​a bug in Meetup's API (hat tip to @casiepa for finding that).

[iandunn]

This is closely related to #2720 and #3321, so I think we should take those into account when considering solutions. There may be an elegant solution which solves all of those cases; whereas coming up with 3 independent solutions could get messy.

[iandunn]

Adding a new unique key for the type and url fields worked.

Props dd32.