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_sqlite.go

@@ -28,7 +28,7 @@ func NewTestDBWithFixture(t testing.TB, _ *sqldb.TestPgFixture) V1Store {
 	store, err := NewSQLStore(
 		&SQLStoreConfig{
 			ChainHash: *chaincfg.MainNetParams.GenesisHash,
-			QueryCfg:  sqldb.DefaultQueryConfig(),
+			QueryCfg:  sqldb.DefaultSQLiteConfig(),
 		}, newBatchQuerier(t),
 	)
 	require.NoError(t, err)