#337 2.1.3/2.1.4 leak sockets when called through evhttp

[AngerMCS]

Libevent 2.1.3 and 2.1.4 (2.1.4 more so) leak sockets when the apps make requests through evhttp.

With 2.1.4 it appears to close NO sockets (even when evhttp_connection_free is called) and the number of sockets open on the system skyrockets rather rapidly.

In 2.1.3 the leak is much slower, but over time sockets will continuously rise and libevent will start to spin in the integrity check for 100% cpu.

2.1.1 and 2.1.2 have no such issues and function as expected.

Example call path that exemplifies this issue:

conn = evhttp_connection_base_new(base, dns_base, host.c_str(), port)
evhttp_make_request(conn, req, method, uri);

later in the callback when done
evhttp_connection_free(conn);