Revert "Revert "More parallellization in the batch importer""

This reverts commit 45b41deb576ca39663cac9dee8cc391421ab764d.
Fix inconsistent behavior in diffsets on remove/add combination.

Previously, remove+add would result in an item added, while add+remove
would result in no net changes. This fixes this inconsistency, leading to net
no changes in both cases.

This resolves two bugs in property and label changing, where one would remove
and add the same label or property multiple times in the same transaction.
Ring changed to SimplePool, which does not rely on acquire/release ordering
Removed IRC link, updated events URL.
Merge pull request #2748 from lutovich/2.1-maint

Fix flaky ParallelBatchImporterTest
Merge pull request #2751 from jakewins/2.1-diffset-correct

Fix inconsistent behavior in diffsets on remove/add combination.
Ensure logging is not shut down after upgrade