storage: defer commit when unlocking a transaction (-> better performance)
Before this change, a storage node did 3 commits per transaction: - once all data are stored - when locking the transaction - when unlocking the transaction The last one is not important for ACID. In case of a crash, the transaction is unlocked again (verification phase). By deferring it by 1 second, we only have 2 commits per transaction during high activity because all pending changes are merged with the commits caused by other transactions. This change compensates the extra commit(s) per transaction that were introduced in commit 7eb7cf1b ("Minimize the amount of work during tpc_finish").
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