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.

I suggest we just keep user relevant info in the CHANGELOG.
Is this valuable to know for end users? Maybe "depends on core 0.82.1" is valuable to developers building Realm from source?