Set SCHED_BATCH priority on the loadblk thread.

While reading another PR I saw a mention of #6358. The use case for
SCHED_BATCH is to hint to the kernel that the thread is running a
non-interactive workload that consumes a lot of CPU time. This is
helpful on desktop machines where the loadblk thread can interfere with
interactive applications. More details can be found in the sched(7) man
page.

src/util.h

@@ -357,4 +357,13 @@ std::unique_ptr<T> MakeUnique(Args&&... args)
     return std::unique_ptr<T>(new T(std::forward<Args>(args)...));
 }
 
+/**
+ * On platforms that support it, tell the kernel the calling thread is
+ * CPU-intensive and non-interactive. See SCHED_BATCH in sched(7) for details.
+ *
+ * @return The return value of sched_setschedule(), or 1 on systems without
+ * sched_setchedule().
+ */
+int ScheduleBatchPriority(void);
+
 #endif // BITCOIN_UTIL_H