Upgrade core library version 0.82.0 -> 0.82.1

This release of core sets `MAX_BPNODE_SIZE=1000` in both release and debug mode. For further details, see https://github.com/Tightdb/tightdb/blob/master/release_notes.md#0821-release-notes.

I have updated CHANGELOG.txt, but I have only added this:

Updating to core library version 0.82.1

I'll let you guys decide on what additional details to add.

Note: This PR includes everything from https://github.com/realm/realm-cocoa/pull/826 as of a few minutes ago.