make BCM_RawDataProvider reentrant

Walter Lamplrequested to merge
wlampl/athena:BCM_RawDataProviderReentrant into master

Motivated by @ishapova's presentation at yesterday's Core Software meeting.

Reviewing the tools used by the BCM_RawDataProvider (BCM_RodDecoder, BCM_RawDataProviderTool) suggests they can be used from a reentrant algo: All const, only mutable atomic counters as private class members.

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