Add store::consume_message, which is like add message but std::move from
the caller such that the messages longer has copies (with
Xapian::Document) on the caller side; this is to avoid threading issues.
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.