[hotfix] use synchronized locks in upstream keystore-saving methods
requested to merge hotfix-restore-synchronized-locks-to-keystore-updating-methods-in-signald into master
- with this MR, we pin our build to this MR of our signald fork: signald-fork@fc58da5a
- it restores behavior to prevent
ConcurrentModificationExceptions when processing a flurry of inbound read receipts