Decrease MAX_VERIFIERS

In current structure, multithreading helps little. Because the verification step should be sequential.

Author: HoOngEe

core/src/verification/queue/mod.rs

@@ -37,7 +37,7 @@ const MIN_MEM_LIMIT: usize = 16384;
 const MIN_QUEUE_LIMIT: usize = 512;
 
 // maximum possible number of verification threads.
-const MAX_VERIFIERS: usize = 8;
+const MAX_VERIFIERS: usize = 2;
 
 /// Type alias for block queue convenience.
 pub type BlockQueue = VerificationQueue<kind::Blocks>;