clang-tidy: Fix performance-unnecessary-value-param check

See: https://clang.llvm.org/extra/clang-tidy/checks/performance/unnecessary-value-param.html

Author: hebasto

include/mp/proxy-io.h

@@ -63,7 +63,7 @@ struct ProxyClient<Thread> : public ProxyClientBase<Thread, ::capnp::Void>
     ProxyClient(const ProxyClient&) = delete;
     ~ProxyClient();
 
-    void setCleanup(std::function<void()> fn);
+    void setCleanup(const std::function<void()>& fn);
 
     //! Cleanup function to run when the connection is closed. If the Connection
     //! gets destroyed before this ProxyClient<Thread> object, this cleanup
@@ -523,7 +523,7 @@ using ConnThread = ConnThreads::iterator;
 // Retrieve ProxyClient<Thread> object associated with this connection from a
 // map, or create a new one and insert it into the map. Return map iterator and
 // inserted bool.
-std::tuple<ConnThread, bool> SetThread(ConnThreads& threads, std::mutex& mutex, Connection* connection, std::function<Thread::Client()> make_thread);
+std::tuple<ConnThread, bool> SetThread(ConnThreads& threads, std::mutex& mutex, Connection* connection, const std::function<Thread::Client()>& make_thread);
 
 struct ThreadContext
 {

src/mp/proxy.cpp

@@ -281,7 +281,7 @@ bool EventLoop::done(std::unique_lock<std::mutex>& lock)
     return m_num_clients == 0 && m_async_fns.empty();
 }
 
-std::tuple<ConnThread, bool> SetThread(ConnThreads& threads, std::mutex& mutex, Connection* connection, std::function<Thread::Client()> make_thread)
+std::tuple<ConnThread, bool> SetThread(ConnThreads& threads, std::mutex& mutex, Connection* connection, const std::function<Thread::Client()>& make_thread)
 {
     const std::unique_lock<std::mutex> lock(mutex);
     auto thread = threads.find(connection);
@@ -316,7 +316,7 @@ ProxyClient<Thread>::~ProxyClient()
     }
 }
 
-void ProxyClient<Thread>::setCleanup(std::function<void()> fn)
+void ProxyClient<Thread>::setCleanup(const std::function<void()>& fn)
 {
     assert(fn);
     assert(!m_cleanup_it);