Make retries configurable

[smarr]

Currently, we do 3 retries when a benchmark fails.
This is really useful for racy implementations.

Though, it's just a time waster for most predictable sequential benchmarks.

The number of retries be an option in the configuration file, and as a command line parameter.

[smarr]

I decided against making it a command-line parameter, in the interest of having a complete documentation of the experiments.

[smarr]

Implemented with #108.