tools: keep tools running even when there are no immediate operations to process.

Change-Id: I47cf15745f2d6f555cab2066d5c2916cf85b843a

tools/dnc/dnc.cpp

@@ -239,6 +239,7 @@ Dnc::Dnc(dht::crypto::Identity identity,
 void
 Dnc::run()
 {
+    auto work = asio::make_work_guard(*ioContext);
     ioContext->run();
 }
 

tools/dsh/dsh.cpp

@@ -256,6 +256,7 @@ dhtnet::Dsh::Dsh(dht::crypto::Identity identity,
 void
 dhtnet::Dsh::run()
 {
+    auto work = asio::make_work_guard(*ioContext);
     ioContext->run();
 }
 

tools/dvpn/dvpn.cpp

@@ -371,6 +371,7 @@ dhtnet::DvpnClient::DvpnClient(dht::InfoHash peer_id,
 void
 dhtnet::Dvpn::run()
 {
+    auto work = asio::make_work_guard(*ioContext);
     ioContext->run();
 }