[treeplayer] Construct TThreadedObject after enabling IMT:

Before, the TThreadedObject had to guess 64 slots were enough for the use case of TTreeProcessorMT - and with 256 cores that was not the case.
Be explicit about the use of IMT for TThreadedObject by providing the new argument.
IMT is enabled before the construction of the TThreadedObject (and thus TThreadedObject now knows how many slots to expect), because the pool is constructed before.
If the order ever gets changes, the TThreadedObject will complain as it is now told to expect IMT usage.

A future TTreeProcessorMT ctor overload might provide a specific thread count, and can pass it down to TThreadedObject.