Fixes #11775: Skip clearing cache when handling new objects

[jeremystretch]

Fixes: #11775

When receiving post_save, check whether the object is newly created. If so, skip deleting any pre-existing cache records.

[kkthxbye-code]

I tried it out and can confirm that it has the same effect on device creation time as adding the index. In general the change just makes sense and will probably improve query time across the board for all creations. I think this is an acceptable first step, if you want you can merge and resolve #11775 - and I'll create a new issue if I find other improvements.

Thanks for handling this :)

[jeremystretch]

Sounds good. I've just opened #11790 to perform an application-wide audit for missing GFK indexes.