Introduced extra parameter to ThreadPoolSvc initPool to allow increase the max allowed parallelism of TBB.

As discussed in https://its.cern.ch/jira/browse/ATR-26285 I added an additional parameter to initPool of the ThreadPoolSvc to allow for increasing the maximum allowed parallelism of TBB. This is set from the new maxParallelismExtra Gaudi property.

In order to do that I had to modify also the IThreadPoolSvc interface (would be great if someone could confirm if it is fine to modify it).

Tagging @fwinkl and @bwynne