SET LOCAL outside a transaction block makes Postgres log "SET LOCAL can only
be used in transaction blocks" on every lock acquisition, even though it works
for the rest of the multi-statement command (madelson#90). set_config(..., true)
behaves the same way but does not log the warning. The calls are wrapped in a
DO block so that the acquire command still returns the lock result as its
first result set.

Adds a PostgresBehaviorTest case showing that set_config(..., true) behaves
like SET LOCAL in the repro from madelson#90.

Fixes madelson#289

…-instead-of-set-local

Use set_config instead of SET LOCAL to avoid Postgres warnings

Fix madelson#296

Release 2.8.4