db/hints: Prefer nested namespaces

This reduces the amount of boilerplate.

db/hints/manager.cc

@@ -46,8 +46,7 @@
 
 using namespace std::literals::chrono_literals;
 
-namespace db {
-namespace hints {
+namespace db::hints {
 
 using namespace internal;
 
@@ -504,5 +503,4 @@ future<> directory_initializer::ensure_rebalanced() {
     });
 }
 
-}
-}
+} // namespace db::hints

db/hints/manager.hh

@@ -38,14 +38,13 @@ class fragmented_temporary_buffer;
 
 namespace utils {
 class directories;
-}
+} // namespace utils
 
 namespace gms {
 class gossiper;
-}
+} // namespace gms
 
-namespace db {
-namespace hints {
+namespace db::hints {
 
 /// A helper class which tracks hints directory creation
 /// and allows to perform hints directory initialization lazily.
@@ -325,5 +324,4 @@ public:
     }
 };
 
-}
-}
+} // namespace db::hints