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.
use unsigned int to avoid having to range check negative numbers,
use const where possible, don't needlessly create empty objects,
use std::move where possible
If an exception is thrown, it is ignored. While this may hide
a bug, this should only be system exceptions in boost, which
is pretty unlikely. Morever, wait should be called manually
before the dtor anyway. Add an error message if the dtor has
to wait in case some such cases creep in so they get fixed.
Coverity 182538
