Use ConcurrentHashMap for timetableList

Multiple requests may be processed at the same time and could otherwise cause problems (concurrent write operations)

src/main/kotlin/org/mosad/thecitadelofricks/controller/CacheController.kt

@@ -33,9 +33,9 @@ import org.slf4j.Logger
 import org.slf4j.LoggerFactory
 import java.io.*
 import java.util.*
+import java.util.concurrent.ConcurrentHashMap
 import java.util.concurrent.Executors
 import kotlin.collections.ArrayList
-import kotlin.collections.HashMap
 import kotlin.collections.HashSet
 import kotlin.concurrent.scheduleAtFixedRate
 import kotlin.time.Duration
@@ -53,7 +53,7 @@ class CacheController {
 
         var courseList = CoursesList(CoursesMeta(), sortedMapOf())
         var mensaMenu = MensaMenu(MensaMeta(0, ""), MensaWeek(), MensaWeek())
-        var timetableList = HashMap<String, TimetableCourseWeek>() // this list contains all timetables
+        var timetableList = ConcurrentHashMap<String, TimetableCourseWeek>() // this list contains all timetables
 
         /**
          * get a timetable, since they may not be cached, we need to make sure it's cached, otherwise download