add missing Boost Thread join_all() call during shutdown

[Diapolo]

• fixes bitcoind shutdown code missing Boost thread join_all()? #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

[gavinandresen]

How did you test the if (!fRet) case (that is, the case of an error during the startup process)?

I tested an alternative version of this patch that just does a join_all in the clean shutdown case by starting up / shutting down 100 times, so ACK on that part.

But I'm not willing to spend the time testing all of the startup-failure cases to make sure they don't result in a hang due to some thread-blocking-waiting-for-another-thread-during-startup case.

[Diapolo]

Gavin, I didn't test them, as this patch is based on an observation (missing join_all()) and your comment in my issue-ticket. I'm fine with removing it for if (!fRet) and just placing a comment there instead.

[Diapolo]

Updated based on Gavins comments.

[gavinandresen]

Nit: it's not true we "can't" test all of the startup failure modes, I actually DID test them all (by simulating all the failure cases with hacked versions of the code) when I did the thread work originally.

Absolutely clean shutdown when there is an error starting up is just too low a priority to justify taking all that time.

[Diapolo]

Updated comment, to address the nit :).

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