Upgrade core library version 0.80.4 -> 0.80.5

This release of core brings a number of bugfixes and some new query related features. For details, see https://github.com/Tightdb/tightdb/blob/master/release_notes.md#0805-release-notes.

I have updated `CHANGELOG.txt`, but I have only added this:

> Updating to core library version 0.80.5

I'll let you guys decide on what additional details from https://github.com/Tightdb/tightdb/blob/master/release_notes.md#0805-release-notes should be reflected in `CHANGELOG.txt`, and how.