Add option for regular stats file updates

[kyakdan]

Honggfuzz updates the stats file only when an increase in coverage (edge, pc, cmp, hw) counters is observed. The stats file includes metrics such as total_exec and exec_per_sec that are interesting to observe regularly to programmatically monitor the progress of a fuzzing run when honggfuzz is run in a pipeline or from a tool that observes the stats file. This PR adds an option to specify an interval in seconds to update the stats file regardless of coverage increases. This is similar to the AFL_FUZZER_STATS_UPDATE_INTERVAL environment variable for AFL++.

More specifically, the responsible function fuzz_perfFeedback is adjusted as follows:

• If there is no coverage increase and the input is imported, then remove this useless input from the corpus
• If there is a coverage increase, log the progress line using LOG_I(...)
• If there is a coverage increase or the update interval has elapsed, update the stats file.
• Finally, if there is a coverage increase, add the input to the corpus and update the coverage metrics.