Upgrade core library version 0.82.2 -> 0.82.3

This is a bugfix release of the core library. For further details, see https://github.com/Tightdb/tightdb/blob/master/release_notes.md#0823-release-notes.

I have updated CHANGELOG.txt, but I have only added this:

Updating to core library version 0.82.3

I'll let you guys decide on what additional details to add.