getlantern/lantern#1895 Stopping old connection pool when restarting ser...

...ver
@myleshorton While fixing 1895, I realized that we were building a new tls.Config on every dial.  This has two downsides:

1. We don't benefit from TLS session caching, instead just creating caches that never actually get used
2. We are parsing the PEM-encoded CA certificate on every dial

To fix this, I'm caching the tls.Configs on a per-masquerade basis.  This should help quite a bit with CPU usage.