lib/nfs/Glue: destruct the NfsManager in the I/O thread

This allows eliminating the indirection code from the NfsConnection
destructor.

src/lib/nfs/Connection.cxx

@@ -22,9 +22,9 @@
 #include "Lease.hxx"
 #include "Domain.hxx"
 #include "Callback.hxx"
+#include "event/Loop.hxx"
 #include "system/fd_util.h"
 #include "util/Error.hxx"
-#include "event/Call.hxx"
 
 extern "C" {
 #include <nfsc/libnfs.h>
@@ -123,14 +123,13 @@ events_to_libnfs(unsigned i)
 
 NfsConnection::~NfsConnection()
 {
+	assert(SocketMonitor::GetEventLoop().IsInside());
 	assert(new_leases.empty());
 	assert(active_leases.empty());
 	assert(callbacks.IsEmpty());
 
 	if (context != nullptr)
-		BlockingCall(SocketMonitor::GetEventLoop(), [this](){
 		DestroyContext();
-			});
 }
 
 void

src/lib/nfs/Connection.hxx

@@ -119,6 +119,9 @@ public:
 	}
 #endif
 
+	/**
+	 * Must be run from EventLoop's thread.
+	 */
 	~NfsConnection();
 
 	gcc_pure

src/lib/nfs/Glue.cxx

@@ -21,6 +21,7 @@
 #include "Glue.hxx"
 #include "Manager.hxx"
 #include "IOThread.hxx"
+#include "event/Call.hxx"
 #include "util/Manual.hxx"
 
 static Manual<NfsManager> nfs_glue;
@@ -43,7 +44,7 @@ nfs_finish()
 	if (--in_use > 0)
 		return;
 
-	nfs_glue.Destruct();
+	BlockingCall(io_thread_get(), [](){ nfs_glue.Destruct(); });
 }
 
 NfsConnection &