Wait for realtime connections after node upgrades in replication tests

Prior to Riak 1.4.8 replication registers as a service prior to completing all
initialization tasks including establishing realtime connections to sink
clusters. This leads to a race condition in the replication_upgrade and
replication2_upgrade tests where the test may begin writing data to the source
cluster to verify the function of realtime replication before the most
recently upgraded node establishes a connection to the sink cluster. The
result of this is that the data is silently discarded by the realtime
replication system and the test fails because all of the expected data is not
replicated and able to be read on the sink cluster. Change the 
replication_upgrade and replication2_upgrade tests to explicitly wait for the
realtime connection to be established after each source cluster node is
upgraded before proceeding with the test.