added "tasks_proof" statistics, via slighly odd global reference Future.forked_proofs (NB: Future.report_status is intertwined with scheduler thread);

Isabelle-hg: http://isabelle.in.tum.de/repos/isabelle@50974 55f8bd61b029824683f8f4970338623401cd2bb7
more systematic task statistics;

Isabelle-hg: http://isabelle.in.tum.de/repos/isabelle@50975 73ec6ad6700ec00d7c7e29d074b1666bbbe31b3f