Remove assertion usage in the Backoff class

[ppatierno]

The current BackOff class implementation uses assert for checking parameter values in the constructor.
It means that TC and CC need to be run with -ea option on the command line.
In order to do that, it's better having the checking logic encapsulated into the code not relying on "external" options.
The issue is about moving away from assert usage and having if / throw checks instead.
At same time it's needed to amend the check-style rules.