Make AccessGuard use a shared_mutex instead (#2702)

This allows `accessConst` to use a shared lock instead of a unique lock

tests/CMakeLists.txt

@@ -36,6 +36,7 @@ set(chatterino_SOURCES
 
 set(test_SOURCES
     ${CMAKE_CURRENT_LIST_DIR}/src/main.cpp
+    ${CMAKE_CURRENT_LIST_DIR}/src/AccessGuard.cpp
     ${CMAKE_CURRENT_LIST_DIR}/src/NetworkCommon.cpp
     ${CMAKE_CURRENT_LIST_DIR}/src/NetworkRequest.cpp
     ${CMAKE_CURRENT_LIST_DIR}/src/UsernameSet.cpp