Remove duplicate properties during store migration from 2.1 to 2.2

Signed-off-by: Jakub Wieczorek <jakub@jakub.cc>
More complicated test data for the duplicate property removal on store migration
Further progress on simplifying the duplicate property fix-up in 2.1-2.2 migration
Addressing minor nits
Add a test for the NodeStore#scanRecords() method
Fixes for failing tests caused by the property deduplication during migration.
Add some sanity checking assertions to the property deduplication.
Address a number of low-risk review comments on the property deduplication.
Merge pull request #3477 from chrisvest/2.2-dupprop-migrate

Remove duplicate properties during store migration from 2.1 to 2.2