Work around bug in VS2015

There seems to be a bug in Visual Studio 2015 that leads to a call tp
logger_impl::set_stream instead of standard_logger_impl::set_stream in
the "basic logging support" test case, which throws a "not supported"
exception instead of actually setting the log stream, leading to a test
case failure. This hasn't been observed for any other compiler,
including more recent versions of Visual Studio. Thus, it was concluded
that this was indeed a compiler bug in VS2015.

Moving the standard_logger_impl implementation out of the anonymous
namespace works around this bug without affecting the behavior of the
library in any way.

src/core/session.cpp

@@ -15,9 +15,6 @@
 using namespace soci;
 using namespace soci::details;
 
-namespace // anonymous
-{
-
 void ensureConnected(session_backend * backEnd)
 {
     if (backEnd == NULL)
@@ -70,8 +67,6 @@ class standard_logger_impl : public logger_impl
     std::string lastQuery_;
 };
 
-} // namespace anonymous
-
 session::session()
     : once(this), prepare(this),
       logger_(new standard_logger_impl),