Fix uninitialized atomic variables

[promag]

This PR fixes uninitialized atomic variables that caused some errors when running with valgrind:

==30439== Thread 5 bitcoin-scheduler:
==30439== Conditional jump or move depends on uninitialised value(s)
==30439==    at 0x6C8E0F: MaybeCompactWalletDB() (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x1863F3: std::_Function_handler<void (), void (*)()>::_M_invoke(std::_Any_data const&) (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x187D38: std::function<void ()>::operator()() const (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x5BEC84: Repeat(CScheduler*, std::function<void ()>, long) (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x5C3683: void boost::_bi::list3<boost::_bi::value<CScheduler*>, boost::_bi::value<std::function<void ()> >, boost::_bi::value<long> >::operator()<void (*)(CScheduler*, std::function<void ()>, long), boost::_bi::list0>(boost::_bi::type<void>, void (*&)(CScheduler*, std::function<void ()>, long), boost::_bi::list0&, int) (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x5C2ACF: boost::_bi::bind_t<void, void (*)(CScheduler*, std::function<void ()>, long), boost::_bi::list3<boost::_bi::value<CScheduler*>, boost::_bi::value<std::function<void ()> >, boost::_bi::value<long> > >::operator()() (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x5C1AAE: std::_Function_handler<void (), boost::_bi::bind_t<void, void (*)(CScheduler*, std::function<void ()>, long), boost::_bi::list3<boost::_bi::value<CScheduler*>, boost::_bi::value<std::function<void ()> >, boost::_bi::value<long> > > >::_M_invoke(std::_Any_data const&) (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x187D38: std::function<void ()>::operator()() const (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x5BE914: CScheduler::serviceQueue() (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x1A2239: boost::_mfi::mf0<void, CScheduler>::operator()(CScheduler*) const (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x19A233: void boost::_bi::list1<boost::_bi::value<CScheduler*> >::operator()<boost::_mfi::mf0<void, CScheduler>, boost::_bi::list0>(boost::_bi::type<void>, boost::_mfi::mf0<void, CScheduler>&, boost::_bi::list0&, int) (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x1907EB: boost::_bi::bind_t<void, boost::_mfi::mf0<void, CScheduler>, boost::_bi::list1<boost::_bi::value<CScheduler*> > >::operator()() (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==  Uninitialised value was created by a heap allocation
==30439==    at 0x4C2E19F: operator new(unsigned long) (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==30439==    by 0x664514: CWallet::CreateWalletFromFile(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >) (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x6673A5: CWallet::InitLoadWallet() (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x170E83: AppInitMain(boost::thread_group&, CScheduler&) (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x13E005: AppInit(int, char**) (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439==    by 0x13E6B6: main (in /home/joao/Projects/bitcoin/src/bitcoind)
==30439== 

==30439== HEAP SUMMARY:
==30439==     in use at exit: 2,384 bytes in 11 blocks
==30439==   total heap usage: 101,957 allocs, 101,946 frees, 69,306,991 bytes allocated
==30439== 
==30439== LEAK SUMMARY:
==30439==    definitely lost: 160 bytes in 1 blocks
==30439==    indirectly lost: 0 bytes in 0 blocks
==30439==      possibly lost: 0 bytes in 0 blocks
==30439==    still reachable: 2,224 bytes in 10 blocks
==30439==         suppressed: 0 bytes in 0 blocks
==30439== Rerun with --leak-check=full to see details of leaked memory
==30439== 
==30439== For counts of detected and suppressed errors, rerun with: -v
==30439== ERROR SUMMARY: 7 errors from 1 contexts (suppressed: 0 from 0)

[theuni]

utACK 4652791

[jonasschnelli]

Kicked travis (issue seems unrelated).
utACK 4652791

[paveljanik]

utACK 4652791

[gmaxwell]

utACK

[TheBlueMatt]

utACK 4652791