Test for fullsync's successful_exists status item

* After a fullsync, make sure that the numner of successful exists equals the number of partitions, since each partition should have one successful fullsync source process exit.
* We could extend this test to handle the parts of replication2 that have down nodes in the future.
* Added a utility function to get the number of partitions from a node, plus a function to get fullsync status items.
* Test passes with changes from branch cet-successful-exists-fix

Requires PR https://github.com/basho/riak_repl/pull/310