Add Cancel function in TTaskGroup.

[zzxuanyuan]

This PR is related to #1010

I could be more efficient to cancel the tasks immediately than wait for unnecessary tasks to be finished. Especially when cache is invalid, unzipping task has to continue running return function.

[phsft-bot]

Can one of the admins verify this patch?

[pcanal]

@phsft-bot build

[phsft-bot]

Starting build on centos7/gcc49, mac1012/native, slc6/gcc49, slc6/gcc62, slc6/gcc62, ubuntu14/native, ubuntu14/native with flags -Dvc=OFF -Dimt=ON -Dccache=ON
How to customize builds

[pcanal]

@phsft-bot build please ... that was too fast last time

[phsft-bot]

Starting build on centos7/gcc49, mac1012/native, slc6/gcc49, slc6/gcc62, slc6/gcc62, ubuntu14/native, ubuntu14/native with flags -Dvc=OFF -Dimt=ON -Dccache=ON
How to customize builds

[phsft-bot]

Build failed on mac1012/native.
See console output.

Warnings:

• /cvmfs/sft.cern.ch/lcg/releases/R/3.2.5-c8cca/x86_64-mac1012-clang81-opt/lib/R/include/R.h:40:10: warning: non-portable path to file '<RConfig.h>'; specified path differs in case from file name on disk [-Wnonportable-include-path]
• /cvmfs/sft.cern.ch/lcg/releases/R/3.2.5-c8cca/x86_64-mac1012-clang81-opt/lib/R/include/R_ext/RS.h:29:10: warning: non-portable path to file '<RConfig.h>'; specified path differs in case from file name on disk [-Wnonportable-include-path]
• /cvmfs/sft.cern.ch/lcg/releases/R/3.2.5-c8cca/x86_64-mac1012-clang81-opt/lib/R/include/Rinternals.h:60:11: warning: non-portable path to file '<RConfig.h>'; specified path differs in case from file name on disk [-Wnonportable-include-path]
• /cvmfs/sft.cern.ch/lcg/releases/R/3.2.5-c8cca/x86_64-mac1012-clang81-opt/lib/R/library/Rcpp/include/Rcpp/r/headers.h:57:10: warning: non-portable path to file '<RVersion.h>'; specified path differs in case from file name on disk [-Wnonportable-include-path]
• /cvmfs/sft.cern.ch/lcg/releases/R/3.2.5-c8cca/x86_64-mac1012-clang81-opt/lib/R/include/R_ext/Visibility.h:29:10: warning: non-portable path to file '<RConfig.h>'; specified path differs in case from file name on disk [-Wnonportable-include-path]
• /cvmfs/sft.cern.ch/lcg/releases/R/3.2.5-c8cca/x86_64-mac1012-clang81-opt/lib/R/include/R.h:40:10: warning: non-portable path to file '<RConfig.h>'; specified path differs in case from file name on disk [-Wnonportable-include-path]
• /cvmfs/sft.cern.ch/lcg/releases/R/3.2.5-c8cca/x86_64-mac1012-clang81-opt/lib/R/include/R_ext/RS.h:29:10: warning: non-portable path to file '<RConfig.h>'; specified path differs in case from file name on disk [-Wnonportable-include-path]
• /cvmfs/sft.cern.ch/lcg/releases/R/3.2.5-c8cca/x86_64-mac1012-clang81-opt/lib/R/include/Rinternals.h:60:11: warning: non-portable path to file '<RConfig.h>'; specified path differs in case from file name on disk [-Wnonportable-include-path]
• /cvmfs/sft.cern.ch/lcg/releases/R/3.2.5-c8cca/x86_64-mac1012-clang81-opt/lib/R/library/Rcpp/include/Rcpp/r/headers.h:57:10: warning: non-portable path to file '<RVersion.h>'; specified path differs in case from file name on disk [-Wnonportable-include-path]
• /cvmfs/sft.cern.ch/lcg/releases/R/3.2.5-c8cca/x86_64-mac1012-clang81-opt/lib/R/include/R_ext/Visibility.h:29:10: warning: non-portable path to file '<RConfig.h>'; specified path differs in case from file name on disk [-Wnonportable-include-path]

And 10 more

Failing tests:

• projectroot.roottest.root.html.roottest_root_html_runMakeIndex