712d35bc56 wallet: Add missing cs_db lock (João Barbosa)
Pull request description:
Without this lock `BerkeleyEnvironment::~BerkeleyEnvironment` and `GetWalletEnv` would race for `g_dbenvs`. This wasn't detected before because thread safety analysis does not check constructors
and destructors.
Reference: http://releases.llvm.org/5.0.2/tools/clang/docs/ThreadSafetyAnalysis.html#no-checking-inside-constructors-and-destructors
Tree-SHA512: 350cb2b991ca699a6bca85f87c82c38f0814484c8ccb0d7d83cb3bff9afcf60dd32b2a9554a9e72eb5803bfad8b6970fe7da618b39be5889178b86faa1b74124