cleanup: Use scoped initializers; move implementation to source file

Simplify code and address comments from
http://gerrit.openbmc-project.xyz/c/openbmc/phosphor-ipmi-blobs/+/27399:

* Use C++17 syntax of intializers in conditionals to simplify code.
* Move getActionHandle() implementation to .cpp
* return false explicitly

Tested:
  Unit tests still pass.

Signed-off-by: Kun Yi <kunyi731@gmail.com>
Change-Id: I08a78125206f284d8c748822912e11828fb39968
diff --git a/manager.hpp b/manager.hpp
index cba7935..3097201 100644
--- a/manager.hpp
+++ b/manager.hpp
@@ -256,16 +256,7 @@
      */
     GenericBlobInterface* getActionHandle(
         uint16_t session,
-        uint16_t requiredFlags = std::numeric_limits<uint16_t>::max())
-    {
-        if (auto item = sessions.find(session);
-            item != sessions.end() && (item->second.flags & requiredFlags))
-        {
-            item->second.lastActionTime = std::chrono::steady_clock::now();
-            return item->second.handler;
-        }
-        return nullptr;
-    }
+        uint16_t requiredFlags = std::numeric_limits<uint16_t>::max());
 
     /**
      * Given a session id will return associated path.