Refactor and modernize namespace declarations

Move all unit tests to namespace nfd::tests

Delete unused header core/algorithm.hpp

Change-Id: I5591f0c5f3bb5db67f8b45fae95471f8a555ca68
diff --git a/daemon/fw/retx-suppression-exponential.cpp b/daemon/fw/retx-suppression-exponential.cpp
index 6158120..6d8cdcb 100644
--- a/daemon/fw/retx-suppression-exponential.cpp
+++ b/daemon/fw/retx-suppression-exponential.cpp
@@ -26,8 +26,7 @@
 #include "retx-suppression-exponential.hpp"
 #include "algorithm.hpp"
 
-namespace nfd {
-namespace fw {
+namespace nfd::fw {
 
 namespace {
 
@@ -143,5 +142,4 @@
   return make_unique<RetxSuppressionExponential>(Duration(init), Duration(max), mult);
 }
 
-} // namespace fw
-} // namespace nfd
+} // namespace nfd::fw