qa/rpc_tests: Wait for handshake to complete in connect_nodes

This avoids a race condition in which the connection was
made but the version handshake is not completed yet. In that
case transactions won't be broadcasted to a peer yet, and
the nodes will wait forever for their mempools to sync.
Merge pull request #4394 from laanwj/2014_06_rpc_tests_connectnode

qa/rpc_tests: Wait for handshake to complete in connect_nodes