We always start a transaction before processing, but there are cases where
we don't need to. Switch to doing it on-demand.
This doesn't make a big difference for sqlite3, but it can for Postgres because
of the latency: 12% or so. Every bit helps!
30 runs, min-max(mean+/-stddev):
Postgres before: 8.842773-9.769030(9.19531+/-0.21)
Postgres after: 8.007967-8.321856(8.14172+/-0.066)
sqlite3 before: 7.486042-8.371831(8.15544+/-0.19)
sqlite3 after: 7.973411-8.576135(8.3025+/-0.12)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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