Exponential backoff int64 overflow

The following line in ComputeBackoff can cause a Int64 overflow:
https://github.com/Azure/azure-functions-host/blob/b44799489586e1912877160e3cfee2a276463a37/src/WebJobs.Script/Utility.cs#L123

So far i only noticed it locally, but it can probably happen on azure itself as well!

Open src/WebJobs.Script/Utility.cs at the linked line and inspect ComputeBackoff, focusing on how exponential growth can exceed Int64 limits. Reproduce the overflow with sufficiently large backoff inputs and verify that the completed change prevents the overflow while preserving expected backoff behavior.

Written by the indexing model from the issue text.