Fix race conditions and fullsync failures in replication test

Fix some race conditions in the cluster leader helper functions. Also
re-initiate fullsync after a certain number of checks for
completion. V2 replication has problems where calling
riak_repl_console:start_fullsync is basically ignored and needs to be
retried.
Adapt to use new clustering API.
Merge branch 'bugfix/replication-test-failures'