Small improvement for background pool in Keeper

src/Coordination/KeeperServer.cpp

@@ -368,7 +368,10 @@ void KeeperServer::launchRaftServer(const Poco::Util::AbstractConfiguration & co
         LockMemoryExceptionInThread::removeUniqueLock();
     };
 
-    asio_opts.thread_pool_size_ = getNumberOfPhysicalCPUCores();
+    /// At least 16 threads for network communication in asio.
+    /// asio is async framework, so even with 1 thread it should be ok, but
+    /// still as safeguard it's better to have some redundant capacity here
+    asio_opts.thread_pool_size_ = std::max(16U, getNumberOfPhysicalCPUCores());
 
     if (state_manager->isSecure())
     {