Add exponential backoff for ScriptHostManager error/restart

[mathewc]

The current logic we have in place for restarts is overly simple and suboptimal (code here). We currently use a constant interval (defaults to 5 seconds, configurable via ScriptHostConfiguration.RestartInterval.

We should put a simple linear (or perhaps exponential) backoff algorithm in place, e.g. starts at 5 seconds and increases 10, 15, etc. to a maximum of 1 minute. We'd then rename the knob to ScriptHostConfiguration.MinRestartInterval. Whenever a host level restart is triggered via a file event, etc. we should reset this interval and break out of any in progress wait. That ensures that we remain responsive during development, while optimizing for production steady state transient failures.