[TreeProcMT] Do not EnableImplicitMT when running event loop
[TreeProcMT] Do not EnableImplicitMT when running event loop

Before this patch, TTreeProcessorMT::Process activated implicit
multi-threading features (and never disabled them). One undesired
side-effect was that ROOT's thread-pool was not destructed after
TTreeProcessorMT was destructed. After this patch, TTreeProcessorMT
multi-thread TTree processing and activation of implicit
multi-threading features are disentangled, and users are
required to call EnableImplicitMT explicitly to activate IMT.
This is consistent with TThreadExecutor's behavior.
This fixes ROOT-10656.

olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,karies/root,olifre/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,karies/root,root-mirror/root,karies/root,karies/root,karies/root,olifre/root,olifre/root,olifre/root,karies/root,root-mirror/root,olifre/root,olifre/root