Add Exponential Backoff to Domain Socket Connection Attempts

[copperlight]

When there is a start ordering issue in Titus between atlas-titus-agent and spectatord, there are 50 or 60 Unable to connect to /run/spectatord/spectatord.unix log messages generated within a second or two. The service recovers eventually when spectatord starts up. Some work is planned to improve Titus system service start ordering, but it would be nice to add backoff to atlas-titus-agent.