Wait for transfers to complete in replication2_pg:test_pg_proxy

Wait for transfers to complete in `replication2_pg:test_pg_proxy`. Replication tests that test the `n_val=1` request option can fail with `insufficient_vnodes` errors if the cluster setup does not include waiting for transfers to complete. Change the `test_pg_proxy` test case to wait until transfers complete on the "A" and "B" clusters before proceeding.

This is intended to address the beta giddyup failure of `replication2_pg:test_basic_pg_mode_mixed_ssl `, but probably affects a good portion of the replication `proxy_get` tests.