fix flaky test TestIdleSweepIgnoresConnectionsWithCalls (#873)
fix flaky test TestIdleSweepIgnoresConnectionsWithCalls (#873)

TestIdleSweepIgnoresConnectionsWithCalls currently flakes for two reasons:
- The ping occasionally fails
- The sweep for client 1 fails for the same reasons as #855

Address the above issues by extending the ping retry timeout and waiting for exchanges to cleanup before checking for connections.
uber/tchannel-go,uber/tchannel-go