add missing Boost Thread join_all() call during shutdown

- fixes #3037 by adding missing join_all() call and brings bitcoind
  shutdown code in line with Bitcoin-Qt shutdown code
- added a comment for the if (!fRet) case
Merge pull request #3059 from Diapolo/Shutdown

add missing Boost Thread join_all() call during shutdown
Re-enable win32 pull-testing
Merge branch 'pulltestwin32'