feat: apply a thread safe lock over dataloaders (#1838)

[samuelAndalon]

cherry pick #1838

[dariuszkuc]

do we need to synchronize any of those methods as well? (assuming no as registry "should be" thread safe?)

    override fun register(key: String, dataLoader: DataLoader<*, *>): DataLoaderRegistry = registry.register(key, dataLoader)
    override fun <K, V> computeIfAbsent(key: String, mappingFunction: Function<String, DataLoader<*, *>>): DataLoader<K, V> = registry.computeIfAbsent(key, mappingFunction)
    override fun combine(registry: DataLoaderRegistry): DataLoaderRegistry = this.registry.combine(registry)
    override fun getDataLoaders(): MutableList<DataLoader<*, *>> = registry.dataLoaders
    override fun getDataLoadersMap(): MutableMap<String, DataLoader<*, *>> = registry.dataLoadersMap
    override fun unregister(key: String): DataLoaderRegistry = registry.unregister(key)
    override fun <K, V> getDataLoader(key: String): DataLoader<K, V> = registry.getDataLoader(key)
    override fun getKeys(): MutableSet<String> = registry.keys
    override fun dispatchAllWithCount(): Int = registry.dispatchAllWithCount()
    override fun dispatchDepth(): Int = registry.dispatchDepth()
    override fun getStatistics(): Statistics = registry.statistics

[samuelAndalon]

lets start with what we have, if we need a fully concurrent and thread safe registry we can just write our own implementation of the cache map instead of using
https://github.com/graphql-java/java-dataloader/blob/master/src/main/java/org/dataloader/impl/DefaultCacheMap.java