syncrepl cookie should be an opaque value

#23 not-withstanding, listen_for_changes() currently requires a path to an existing file (see #22). For a new API in 3.1, it should not be tied to a file - it should be the responsibility of the calling application to persist the sync cookie.

In order to maintain feature parity and minimise disruption to API consumers, a Perl-level helper could be provided that implements the same semantics.
