Make `ROOT::EnableThreadSafety();` the default?

[pcanal]

Using threads in conjunction with the ROOT library is now very much common place and more than once users have run into problem 'just because' they forgot to call ROOT::EnableThreadSafety();.

After verifying that the cause on a single thread application is low enough (to not harm 'too much' existing users running large single thread workloads), we probably should make ROOT::EnableThreadSafety(); the default.

[pcanal]

Note: this is related to #11955 which is one example where changing the default would not only affect performance but also have a functional effect (gDirectory having different values before and after).

[dpiparo]

This is a matter to be discussed widely in the team. However, I am also of the opinion that the option should be enabled by default.