neorados: build without "using namespace std"

* add "std::" prefix in headers
* add "using" declarations in .cc files.

so we don't rely on "using namespace std" in one or more included
headers.

Signed-off-by: Kefu Chai <kchai@redhat.com>

src/neorados/cls/fifo.cc

@@ -35,6 +35,8 @@
 
 #include "fifo.h"
 
+using namespace std;
+
 namespace neorados::cls::fifo {
 namespace bs = boost::system;
 namespace cb = ceph::buffer;

src/neorados/cls/fifo.h

@@ -757,7 +757,7 @@ private:
     auto e = ba::get_associated_executor(handler, get_executor());
     auto a = ba::get_associated_allocator(handler);
     _read_meta_(r, oid, ioc,
-		nullopt,
+		std::nullopt,
 		ca::bind_ea(
 		  e, a,
 		  [this,