Avoid long-running transactions. When hivesense is syncing at the same

time as hivemind, it may need to sleep until hivemind has processed
blocks.  We were sleeping with a transaction open, and holding open
a long-running transaction kept the autovacuum system from working,
and that crippled hivemind and reptracker (at least).
Also fix a similar problem with the workers, which were waiting inside
a transaction.