sqldb: demonstrate the use of ExecutePagedQuery

Here, a new query (GetChannelsByOutpoints) is added which makes use of
the /*SLICE:outpoints*/ directive & added workaround. This is then used
in a test to demonstrate how the ExecutePagedQuery helper can be used to
wrap a query like this such that calls are done in pages.

The query that has been added will also be used by live code paths in an
upcoming commit.

sqldb/sqlite_test.go

@@ -7,6 +7,14 @@ import (
 	"testing"
 )
 
+// isSQLite is true if the build tag is set to test_db_sqlite. It is used in
+// tests that compile for both SQLite and Postgres databases to determine
+// which database implementation is being used.
+//
+// TODO(elle): once we've updated to using sqldbv2, we can remove this since
+// then we will have access to the DatabaseType on the BaseDB struct at runtime.
+const isSQLite = true
+
 // NewTestDB is a helper function that creates an SQLite database for testing.
 func NewTestDB(t *testing.T) *SqliteStore {
 	return NewTestSqliteDB(t)