Fix sqlite issues in the "upstream buffer" test. The test used
`:memory:` to have an in-memory DB to separate from other tests. The
"flush at shutdown" code runs in a different goroutine, though, and
creates a new DB connection. The SQLite separation was too strong,
making that function not find any tables. This is now solved by having
an in-memory database that's shared between all connections made from
the same unit test.
da1b42f9fa