lib: move transaction handling to mu-xapian

Instead of handling transactions in the store, handle it in xapian-db.
Make the code a bit more natural / cleaner-out

Handle transaction automatically (with a batch-size) and add some RAII
Transaction object, which makes all database interaction transactable
for the duration. So, no more need for explicit parameters to
add_message while indexing.

lib/tests/test-mu-store-query.cc

@@ -125,7 +125,6 @@ I said: "Aujourd'hui!"
 }};
 	TempDir tdir;
 	auto store{make_test_store(tdir.path(), test_msgs, {})};
-	store.commit();
 
 	// matches
 	for (auto&& expr: {
@@ -846,7 +845,6 @@ https://trac.xapian.org/ticket/719
 
 	TempDir tdir;
 	auto store{make_test_store(tdir.path(), test_msgs, conf)};
-	store.commit();
 
 	/* true: match; false: no match */
 	const auto cases = std::vector<std::pair<std::string_view, bool>>{{