dir, stored: start statistics threads only if needed

[alaaeddineelamri]

Description:

The director statistics thread starts even when not needed. In order to optimize and have less unnecessary workload, the statistics thread for the director is not started by default, unless collect statistics and statistics collect interval are both specified.

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[pstorz]

The code that handles the return status of StartStatisticsThread() in both storage and director needs to be added.
Good work!

[pstorz]

Good work, however I would use M_ERROR_TERM instead of M_WARNING if the statistics thread is required but cannot be started. This probably is a serious problem.

Other than that, the last commit message could be reformatted as it has a single "s" in one line that needs to be removed.

Thank you!

[pstorz]

Good work!