This speeds up initial connections with an old/large addr.dat a lot.
Do not automatically change the wallet format unless the user takes an explicit action that implies an upgrade (encrypting, for now), or uses -walletupgrade. -walletupgrade optionally takes an integer argument: the client version up to which upgrading is allowed. Without an argument, it is upgraded to latest supported version. If an argument to -walletupgrade is provided at the time the wallet is created, the new wallet will initially not use features beyond that version. Third, the current wallet version number is reported in getinfo.
It could just as well be on either part of the gitian build, but to safely put it on the Qt side would require bumping the filename, and every gitian user rebuilding it. v0.5.3.1 put it on the Bitcoin side, and this is easier to work with and keep safe, so I'm moving it. Use `qmake MINGW_THREAD_BUGFIX=0` to disable