Drop validation threadpool to low pri.

src/chasers/chaser_validate.cpp

@@ -47,7 +47,7 @@ chaser_validate::chaser_validate(full_node& node) NOEXCEPT
     initial_subsidy_(node.config().bitcoin.initial_subsidy()),
     subsidy_interval_(node.config().bitcoin.subsidy_interval_blocks),
     threadpool_(std::max(node.config().node.threads, 1_u32),
-        network::thread_priority::normal)
+        network::thread_priority::low)
 {
 }