Evgeny, you can use "netstat -tnp |grep curl " to get the other port number of the connection from curl to apache2. With that, you can look for the other end of the connection in "netstat -tnp" output. The last column should give be "123/apache2" where 123 is the pid of the apache2 process. You will have to execute netstat -tnp as root to get the info.
The backtrace of the child process you posted looks more like a process that is waiting for a connection. But one would need the debug info installed to be absolutely sure.