This will allow client code to provide more meaningful diagnostics. In
particular it will enable "notmuch new" to continue suggsting the user
run "notmuch setup" to create a config after "notmuch new" is
transitioned to the new configuration framework.
This moves away from the deprecated notmuch_database_add_message API
and instead uses the notmuch_database_index_file API. This means
instroducing a class to manage the index options and bumping the
library version requirement to 5.1.
