Avoids a DB error (leading to an assert) where a thread uses
a read txn previously created with an environment that was
since closed and reopened. While this usually works since
BlockchainLMDB renews txns if it detects the environment has
changed, this will not work if objects end up being allocated
at the same address as the previous instance, leading to stale
data usage.
Thanks hyc for the LMDB debugging.
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