GaudiCommon counters not thread safe after !834

The following discussion from !834 (merged) should be addressed:

• @clemenci started a discussion:

  I think this is what causes the bad counts in multithreaded jobs spot by @cattanem.

  @graven, you correctly protected the new declareCounter method, but you left m_countersOwn unprotected.

  I'll prepare a MR to try to fix it.