Fix an error that can lead to failure of tests using replication2_pg

Fix an error that can lead to failure of tests using
replication2_pg:test_pg_proxy test case. A protocol buffers connection
is established to a node in the "B" cluster, the leader node from that
cluster is shut down, and then that protocol buffers connection is
used to exercise proxy_get. If the connection was established to the
former leader and that is subsequently shut down it can cause the test
to stall and eventually fail. This changes that test to establish a
new connection to a node remaining in the "B" cluster to use for the
proxy_get and prevents the test from stalling.
Merge branch 'bugfix/replication2-pg-test-pg-proxy'