use unique_ptr to network_thread_pool

include/libtorrent/aux_/session_impl.hpp

@@ -760,7 +760,7 @@ namespace libtorrent
 
 			// a thread pool used for async_write_some calls,
 			// to distribute its cost to multiple threads
-			std::vector<boost::shared_ptr<network_thread_pool> > m_net_thread_pool;
+			std::vector<std::unique_ptr<network_thread_pool>> m_net_thread_pool;
 
 			// the bandwidth manager is responsible for
 			// handing out bandwidth to connections that

src/session_impl.cpp

@@ -6083,7 +6083,7 @@ namespace aux {
 		int num_pools = num_threads > 0 ? num_threads : 1;
 		while (num_pools > m_net_thread_pool.size())
 		{
-			m_net_thread_pool.push_back(boost::make_shared<network_thread_pool>());
+			m_net_thread_pool.emplace_back(new network_thread_pool());
 			m_net_thread_pool.back()->set_num_threads(num_threads > 0 ? 1 : 0);
 		}