Compatibility with 7.0.0-SNAPSHOT (develop) performance stats format

[joewiz]

eXist-db/exist#4910 introduced a change to the structure of JMX performance stats returned by the /status endpoint. That change has been merged into 7.0.0-SNAPSHOT (develop) and will not be back-ported to 6.x, so the release with this feature needs to bump its eXist-db version dependency - and thus monex itself will need a new major version.

[duncdrum]

see #261