Add ability to randomize the reconnect backoff time

[nickdaugherty]

In the event of a dropped connection, it is likely that all clients connecting to the same service could experience the outage. For example, a Thrift server that suddenly crashes.

In this case, there will be repeated, (nearly) simultaneous reconnection attempts, which could overload some servers.

A better approach would be to add a small degree of randomization to the backoff period to ensure each connection hits the target server at different times.