graph/db+sqldb: find best default query cfg values for sqlite & postgres

This commit adds a BenchmarkFindOptimalSQLQueryConfig test in the
graph/db package which runs ForEachNode and ForEachChannel queries
against a local backend using various different values for the sql
QueryConfig struct. This is done to determine good default values to
use for the config options for sqlite vs postgres.

graph/db/test_postgres.go

@@ -43,7 +43,7 @@ func NewTestDBWithFixture(t testing.TB,
 	store, err := NewSQLStore(
 		&SQLStoreConfig{
 			ChainHash: *chaincfg.MainNetParams.GenesisHash,
-			QueryCfg:  sqldb.DefaultQueryConfig(),
+			QueryCfg:  sqldb.DefaultPostgresConfig(),
 		}, querier,
 	)
 	require.NoError(t, err)