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/init.cpp

@@ -625,6 +625,7 @@ void ThreadImport(std::vector<fs::path> vImportFiles)
 {
     const CChainParams& chainparams = Params();
     RenameThread("bitcoin-loadblk");
+    ScheduleBatchPriority();
 
     {
     CImportingNow imp;