Logs if awaiting transactions to be closed

Maximum every 30 seconds. Can happen when rotating the log or shutting
down. An issue has been observed where there has been an indefinite wait
for closed transactions to catch up on committed transactions and logging
has been put in to help debugging that issue.
Merge pull request #2974 from tinwelint/2.2-log-shutdown-await-txs

Logs if awaiting transactions to be closed